The White House is "highly skeptical" of suggestions that Syrian rebels used chemical weapons, spokesman Jay Carney said on Monday in response to comments by a UN investigator that rebel forces have used the nerve agent, sarin.
Carney noted that the UN independent commission of inquiry on Syria issued a statement distancing itself from claims by Carla Del Ponte, one of the investigators on the commission.
She said on Sunday that human rights investigators had gathered testimony from casualties of Syria's civil war and medical staff indicating that rebel forces have used sarin.
"We find it highly likely that chemical weapons, if they were in fact used in Syria - and there is certainly evidence that they were - that the Assad regime was responsible," Carney told reporters in a White House briefing, referring to the government of Syrian President Bashar al-Assad.
Assad's government and the rebels accuse each other of carrying out three chemical weapons attacks, one near Aleppo and another near Damascus, both in March, and another near Homs in December.
The United States has said it has "varying degrees of confidence" that sarin has been used by Syria's government on its people.