(CNN) -- The leaders of both parties in the House of Representatives emerged from a White House meeting Tuesday to support President Barack Obama's call for American strikes against government forces in Syria's civil war.

House Speaker John Boehner, R-Ohio, told reporters that only the United States has the "the capability and capacity" to respond to what Washington says was a poison gas attack by troops loyal to Syrian President Bashar al-Assad.

"The use of chemical weapons is a barbarous act. It's pretty clear to me that the United Nations is unable to take action; NATO, not likely to take action," Boehner said.

And House Minority Leader Nancy Pelosi added that Washington must respond to actions "outside the circle of civilized human behavior."

"Humanity drew a line decades ago that I think if we ignore, we do so to the peril of many other people who could suffer," said Pelosi, D-California.

And Boehner's deputy, Majority Leader Eric Cantor, R-Virginia, added in a written statement, "America has a compelling national security interest to prevent and respond to the use of weapons of mass destruction."

Obama met with congressional leaders Tuesday morning to lobby for their support on Syria, promising that American strikes would deliver important results while keeping the United States out of a larger war.

"This is not Iraq, and this is not Afghanistan," Obama told reporters before the meeting. "This is a limited, proportional step that will send a clear message -- not only to the Assad regime, but also to other countries that may be interested in testing some of these international norms -- that there are consequences."

Syria denies the accusations and accuses rebel groups of using chemical weapons, while the rebels blame government troops. The United States and several of its allies say the rebels don't have the capability to launch a large-scale chemical attack like the one seen outside Damascus on August 21.

The United States, along with NATO and several other countries, blames al-Assad's forces for a chemical weapons attack that's believed to have killed more than 1,000 people -- including, Obama said Tuesday, more than 400 children.

"This norm against using chemical weapons -- that 98 percent of the world agrees to -- is there for a reason, because we recognize that there are certain weapons that, when used, can not only end up resulting in grotesque deaths, but also can end up being transmitted to non-state actors, can pose a risk to allies and friends of ours like Israel, like Jordan, like Turkey," Obama said.

Failing to punish Syria would send "a message that international norms around issues like nuclear proliferation don't mean much," he said. The U.S. military plan "gives us the ability to degrade Assad's capabilities when it comes to chemical weapons.

"It also fits into a broader strategy that we have to make sure that we can bring about over time the kind of strengthening of the opposition and the diplomatic and economic and political pressure required so that ultimately we have a transition that can bring peace and stability, not only to Syria, but to the region."

But the threat of a U.S. attack has, as expected, led to "a lot of dispersal" of Syria military assets in recent days, according to a US official with direct access to the latest information. The U.S. military needs to continuously update its target lists, the official said.

And although U.S. Secretary of State John Kerry said Sunday that the prospect of strikes has resulted in about 100 defections from the Syrian military, the U.S. official who spoke to CNN Tuesday said there have been no major or significant defections.

Among the top U.S. lawmakers meeting Tuesday with Obama were House Speaker John Boehner and Minority Leader Nancy Pelosi.

In brief remarks, Boehner said he supports Obama's call for military action. "This is something that the United States as a country needs to do," he told reporters.

Pelosi also expressed support for action, saying al-Assad "crossed a line" with the use of chemical weapons "and we must respond."

Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, Secretary of State John Kerry and Defense Secretary Chuck Hagel also were present.

The White House push comes after al-Assad once again raised the specter of an all-out regional war if the United States strikes.

Syria's allies Russia and China, meanwhile, remain steadfastly opposed to military action, unconvinced by evidence that the United States and France say shows al-Assad's forces used chemical weapons in an August 21 attack around Damascus.

Dempsey, Kerry and Hagel are scheduled to appear before the Senate Foreign Relations Committee later Tuesday to press the case for action against Syria, a senior State Department official said.

Kerry will argue that a failure to act "unravels the deterrent impact of the international norm against chemical weapons use," said the official, who spoke on condition of anonymity.

A senior Pentagon official told CNN Dempsey will focus in his testimony on answering questions from the members about the use of military force, rather than making an overt case for military action. This would be in line with Dempsey's views that the military will be ready to do whatever the president orders.

It will mainly be Hagel who makes the case that the use of force is warranted, a second senior defense official said. Both declined to be identified because they were speaking in advance of the hearing.

Obama is expected to meet with leaders of the key national security committees in the House and Senate.

Administration officials will also be conducting classified briefings on Syria for Congress nearly every day this week.

Israeli missile test

Amid heightened tension in the region, Israel carried out a missile test Tuesday morning in the Mediterranean -- a launch detected by a Russian early warning system before it was confirmed by Israeli authorities, sending ripples of anxiety among observers.

Israel's Ministry of Defense said it and the U.S. Missile Defense Agency "completed a successful flight test (of the) new version of the Sparrow target missile."

The trial was carried out from an Israeli test range over the Mediterranean Sea, it said. The Arrow defense system successfully detected and tracked the system, it added.

Earlier, a Ministry of Defense Facebook post said an Anchor "target" missile was the type tested.

Arieh Herzog, a former head of Israel's missile defense program who was present at the Israeli air force test facility in central Israel when the test was conducted, told CNN it was intended to trial the new Sparrow target missile.

The target missile, launched from an aircraft, simulates a real, incoming, long range missile, he said.

A U.S. official told CNN it was "an expected Israeli system test" in the eastern Mediterranean Sea. The official said it was up to the Israelis to explain exactly what they were testing.

Pentagon spokesman George Little said the United States had "provided technical assistance and support" to the Israeli military for the flight test, in line with regular cooperation on long-range missile defense projects.

"The test was long planned to help evaluate the Arrow Ballistic Missile Defense system's ability to detect, track, and communicate information about a simulated threat to Israel," he said. "This test had nothing to do with United States consideration of military action to respond to Syria's chemical weapons attack."

A U.S. military official was earlier adamant that the event "did not involve U.S. forces."

Five U.S. warships were in the Mediterranean Sea last week.

Ben Goodlad, a senior defense analyst at IHS Jane's, said that a target missile "has similar ballistic characteristics as a missile. However, it does not feature a warhead and is intended to merely simulate a launch in order to test detection, tracking and interception capabilities."

This is likely why there was no reported missile strike, he said. Any missiles fired against Syria from vessels in the Mediterranean would be Tomahawk cruise missiles, he said, "which follow a far flatter trajectory than ballistic missiles and would not be mistaken as such by early warning radar."

Russia's Defense Ministry said it detected the launch of two "ballistic objects" from the Mediterranean Sea toward the eastern Mediterranean coast at 2:16 a.m. ET, Russian news agency RIA-Novosti reported. Defense Minister Sergey Shoigu informed President Vladimir Putin of the development, it added.

Bill Neeley, international editor for CNN affiliate ITN, tweeted from Damascus that there have been "no major explosions in #Damascus area that might have been from missiles fired from Med. Several blasts 2 hrs ago from army to rebel areas."

'A lot of distrust'

In the United States, the Obama administration is expected to increase its lobbying efforts as lawmakers return to Capitol Hill for briefings.

A leading member of one of the key U.S. committees, House Armed Services Committee Chairman Buck McKeon, said the administration will have to overcome "a lot of distrust among the American people" about the intelligence that it says shows Syria's government used chemical weapons.

"There will be a real questioning as to the veracity of the evidence and if this really happened or not," McKeon, R-California, said in an interview with CNN's Barbara Starr. "It will be necessary to explain and prove to the American people, and I think the only person who can really do that is the president of the United States."

The United States and several of its leading allies accuse al-Assad's forces of resorting to poison gas attacks against rebel forces and civilians, including an August 21 attack near Damascus the Obama administration says killed more than 1,400 people.

Obama said Saturday that the use of chemical weapons is "a challenge to the world" that threatens U.S. allies in the region -- but he said he would seek the authorization of Congress before unleashing American force.

No vote on military action is expected until after lawmakers reconvene from recess on September 9.

More support for the opposition

U.S. plans for strikes against Syria may be coupled with increased support for rebel forces in that country's civil war, two leading Republican senators said after meeting with the president on Monday.

Obama huddled in the Oval Office with McCain and Sen. Lindsey Graham, both members of the Senate Armed Services Committee, the White House said. After the meeting, McCain and Graham said the United States needs to help the rebels reverse battlefield gains by troops loyal to al-Assad.

"We still have significant concerns, but we believe there is in formulation a strategy to upgrade the capabilities of the Free Syrian Army and to degrade the capabilities of Bashar al-Assad," said McCain, the ranking Republican on the armed services committee.

McCain, who has called for U.S. intervention in Syria since early 2012, criticized Obama's decision to seek a vote before striking. But he said it would be "catastrophic" for Congress to reject the president's call to authorize military force.

"It would undermine the credibility of the United States of America and the president of the United States," McCain said. "None of us want that."

In an interview with CNN Tuesday, McCain added that he wants Congress to vote for a resolution that is "meaningful, impactful and can shift the balance of power on the ground in Syria."

McCain, a former Republican presidential candidate, said delays to U.S. military strikes are giving the Syrian military time to move its assets into safety and risks putting more civilians in danger.

Graham told CNN that Obama must do more to convince the American people of the need for action on Syria. The senator believes Iran will be emboldened to continue its pursuit of nuclear weapons if no international action is taken against Syria, and the region will be further destabilized.

"If we get Syria right, maybe we can avoid a war between Israel and Iran which we would surely get (dragged) into," he said. "If we fail to stand up for the right thing and send the right message, the whole region will go up in flames."

Lawmaker's 'big question'

The threat of a regional war is an angle that the Syrian president continues to flag.

"The Middle East is a powder keg, and the fire is approaching today," he told French newspaper Le Figaro in an interview Monday.

"One must not speak only of the Syrian response, but rather what could be produced after the first strike," he said. "Because nobody can know what will happen."

Neither Washington nor Paris has "a single proof" that the Syrian government was behind the alleged chemical weapons attack against civilians on August 21, he said.

Obama would have gone back to the U.N. Security Council if he were a strong leader, Assad said, but instead he has given in to pressure to act from within the United States.

Sample analysis

Syria has repeatedly denied being behind the August 21 attack and accuses rebel fighters of using chemical weapons on government troops.

U.N. weapons inspectors left Syria Saturday with evidence that will determine whether poison gas was used in that attack and tests on those samples are being conducted "as fast as it is possible to do within the scientific constraints," said Martin Nesirky, spokesman for Secretary-General Ban Ki-moon.

The United Nations has said more than 100,000 people -- including many civilians -- have been killed in Syria since a popular uprising spiraled into a civil war in 2011. Syrian opposition activists reported another 107 dead on Monday, mostly in Damascus and its suburbs.

Numbers released by the United Nations Tuesday point to the staggering impact the war has had on the nation.

The number of Syrians who have fled their war-ravaged country has risen above 2 million, the U.N. refugee agency reported, an increase of nearly 1.8 million people over the past 12 months.

CNN's Barbara Starr, Matt Smith, Elise Labott, Alla Eshchenko, Michael Schwartz, Evan Perez, Dana Bash, David McKenzie, Ashley Killough, Sarah Chiplin, Khushbu Shah, Mohammed Tawfeeq, Sandrine Amiel and Niki Cook contributed to this report.