The White House says it’s highly likely that Syrian President Bashar Assad’s regime, not the rebel opposition, was behind any chemical weapons use in Syria.

White House spokesman Jay Carney says there is certainly evidence that chemical weapons have been used. But Carney says the U.S. is highly skeptical of claims that rebels put them in play.

On Sunday, a member of a U.N. panel investigating in Syria said there were indications that rebel forces had used the nerve agent sarin.

Carney says the U.S. is still looking for conclusive evidence about chemical weapons use. He says there’s no timeline for the investigation.

Secretary of State John Kerry was leaving for Moscow on Monday to discuss the situation in Syria with Russian President Vladimir Putin.

(AP)