BEIRUT (AP) -- Two days of bloody turmoil in Syria killed more than 50 people as forces loyal to President Bashar Assad shelled residential buildings, fired on crowds and left bleeding corpses in the streets in a dramatic escalation of violence, activists said Friday.

Much of the violence was focused in Homs, where heavy gunfire hammered the city Friday in a second day of chaos. A day earlier, the city saw a flare-up of sectarian kidnappings and killings between its Sunni and Alawite communities, and pro-regime forces blasted residential buildings with mortars and gunfire, according to activists who said an entire family was killed.

Video posted online by activists showed the bodies of five small children, five women of varying ages and a man, all bloodied and piled on beds in what appeared to be an apartment after a building was hit in the Karm el-Zaytoun neighborhood of the city. A narrator said an entire family had been "slaughtered."

The video could not be independently verified.

Activists said at least 30 people were killed in Homs on Thursday and another 21 people were killed across the country Friday.

In an attempt to stop the bloodshed in Syria, the U.N. Security Council was to hold a closed-door meeting Friday to discuss the crisis, a step toward a possible resolution against the Damascus regime, diplomats said.

The Syrian uprising, which began nearly 11 months ago with mostly peaceful protests, has become increasingly violent in recent months as army defectors clash with government forces and some protesters take up arms to protect themselves. The violence has enflamed the potentially explosive sectarian divide in the country, where the Alawite minority dominates the regime despite a Sunni Muslim majority. The U.N. estimates that more than 5,400 people have been killed since March.

The head of Arab League observers in Syria said in a statement that violence in the country has spiked over the past few days. Sudanese Gen. Mohammed Ahmed al-Dabi said the cities of Homs, Hama and Idlib have all witnessed a "very high escalation" in violence since Tuesday.

A "fierce military campaign" was also under way in the Hamadiyeh district of Hama since the early hours of Friday, according to the British-based Syrian Observatory for Human Rights and other activists. They said the sound of heavy machine-gun fire and loud explosions reverberated across the area.

Some activists reported seeing uncollected bodies in the streets of Hama.

Elsewhere, a car bomb exploded Friday at a checkpoint outside the northern city of Idlib, the Observatory said, citing witnesses on the ground. The number of casualties was not immediately clear.

Details of Thursday's wave of killings in Homs were emerging from an array of residents and activists on Friday, though they said they were having difficulty because of continuing gunfire.

"There has been a terrifying massacre," Rami Abdul-Rahman, director of the Syrian Observatory for Human Rights, told the AP on Friday, calling for an independent investigation.

Thursday started with a spate of sectarian kidnappings and killings between the city's population of Sunnis and Alawites, a Shiite sect to which Assad belongs as well as most of his security and military leadership, said Mohammad Saleh, a centrist opposition figure and resident of Homs.

There was also a string of attacks by gunmen on army checkpoints, Saleh said. Checkpoints are a frequent target of dissident troops who have joined the opposition.

The violence culminated with the evening killing of the family, Saleh said, adding that the full details of what happened were not yet clear.

The Observatory said at least 11 people, including eight children, died when a building came under heavy mortar and machine gun fire. Some residents spoke of another massacre that took place when shabiha - armed regime loyalists - stormed the district, slaughtering residents in an apartment, including children.

"It's racial cleansing," said one Sunni resident of Karm el-Zaytoun, speaking on condition of anonymity for fear of reprisal. "They are killing people because of their sect," he said.

Some residents said kidnappers were holding Alawites in the building hit by mortars and gunfire, but the reports could not be confirmed.

Thursday's death toll in Homs was at least 35, said the Observatory and the Local Coordination Committees, an umbrella group of activists. Both groups cite a network of activists on the ground in Syria for their death tolls. The reports could not be independently confirmed.

Syria tightly controls access to trouble spots and generally allows journalists to report only on escorted trips, which slows the flow of information.

The Syrian uprising began last March with largely peaceful anti-government protests, but it has grown increasingly violent in recent months.

Also Friday, Iran's official IRNA news agency said gunmen in Syria have kidnapped 11 Iranian pilgrims traveling by road from Turkey to Damascus.

Iranian pilgrims routinely visit Syria - Iran's closest ally in the Arab world - to pay homage to Shiite holy shrines. Last month, 7 Iranian engineers building a power plant in central Syria were kidnapped. They have not yet been released.

The Free Syrian Army - a group of army defectors - released a video on its Facebook page claiming responsibility for the kidnapping and saying the Iranians were taking part in the suppression of the Syrian people. The leader of the group could not be reached for comment.

Bassma Kodmani, a spokeswoman for the opposition Syrian National Council, said the group is working to help the army defectors to link them up and supply them with everything from communications equipment to clothes. Speaking in Paris, she said defectors are increasingly swelling the ranks of the Free Syrian Army and it is becoming a critical force in the uprising.

In Cairo, around 200 opposition Syrians protested outside the Syrian Embassy, trying to break into the building. They threw stones and bricks at the building, but were kept back by a line of police and soldiers.

Assad's regime claims terrorists acting out a foreign conspiracy are behind the uprising, not protesters seeking change, and that thousands of security forces have been killed.

International pressure on Damascus to end the bloodshed so far has produced few results.

The Arab League has sent observers to the country, but the mission has been widely criticized for failing to stop the violence. Gulf states led by Saudi Arabia pulled out of the mission Tuesday, asking the Security Council to intervene because the Syrian government has not halted its crackdown.

The U.N. Security Council has been unable to agree on a resolution since violence began in March because of strong opposition from Russia and China.

Russian Deputy Foreign Minister Gennady Gatilov said Friday that Moscow will oppose a new draft U.N. resolution on Syria worked out by the West and some Arab states because it does not exclude the possibility of outside military interference.

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