Ban Ki-moon and Bashar al-Assad

(CNN) -- The United Nations will investigate Syria's claim that rebels may have used chemical weapons in the country, U.N. Secretary-General Ban Ki-moon said Thursday.

Opposition groups, meanwhile, have insisted that the Syrian regime itself used such weapons.

U.S. officials have said there is no known evidence to back up the claims on either side.

Syria asked for a U.N. investigation of its claim, and Ban said he has a mandate to consider such a request from any member state. So the U.N. probe will focus on the government's allegation.

"In discharging its mandate of an investigation mission, full cooperation from all parties will be essential," Ban said Thursday. "I stress that this includes unfettered access."

U.N. teams have previously been unable to reach some areas in the ongoing conflict.

Susan Rice, the U.S. ambassador to the United Nations, said the United States welcomes Ban's announcement.

"The United States supports an investigation that pursues any and all credible allegations of the possible use of chemical weapons in Syria, and underscores the importance of launching this investigation as swiftly as possible," she said in a statement.

"We demand the full cooperation of the Assad regime in particular, as well as Syrian authorities throughout the country, including by providing full and unfettered access to all relevant individuals and locations. In addition, humanitarian workers seeking to assist injured individuals should be given complete access to provide medical care and assistance as needed."

She added that U.S. President Barack Obama "has been clear that the use or transfer of chemical weapons is totally unacceptable. If (Syrian President) Bashar Al-Assad and those under his command make the mistake of using chemical weapons, or fail to meet their obligation to secure them, then there will be consequences. Those responsible will be held accountable."

Ban said his teams are consulting with the Organization for the Prohibition of Chemical Weapons and the World Health Organization.

Most members of the U.N. Security Council planned to ask Ban for an inquiry into the reports, according to Gerard Araud, the French ambassador to the United Nations.

Robert Ford, U.S. ambassador to Syria, said Wednesday that while there is no evidence so far to substantiate the claims that chemical weapons were used, U.S. officials are "looking very carefully at these reports. We are consulting with partners in the region and in the international community."

CNN's Richard Roth, Chelsea Carter, Amir Ahmed and Barbara Starr contributed to this report.