U.S. officials said Monday that they were taking seriously new allegations that Syrian rebels had used chemical weapons but had seen no evidence to confirm the reports.

The Obama administration has said it believes the Syrian government likely used the nerve agent sarin in recent months and has supported a U.N. investigation into the use of chemical agents in Syria, where the government of President Bashar al-Assad has alleged the illicit weapons were used by the opposition.

On Sunday night, a Swiss broadcaster aired an interview with a leading member of a U.N. investigative committee, Carla Del Ponte, in which she said there are indications the rebels, not the government, used chemical weapons. U.S. officials, however, said they did not believe the rebels had obtained sarin or other such illicit agents.

“We are highly skeptical of suggestions that the opposition could have or did use chemical weapons,” White House press secretary Jay Carney told reporters. “We find it highly likely that any chemical weapon use that has taken place in Syria was done by the Assad regime. And that remains our position.”

U.S. officials said they were nonetheless taking the report seriously and used it to make the case anew that a U.N. inspection team should be allowed into Syria for on-the-ground testing. The inspectors have been blocked by the Syrian government because of a disagreement over the scope of the inquiry.

That team is not connected to Del Ponte’s panel, the Independent International Commission of Inquiry, which is investigating possible rights violations in Syria. On Monday, the panel sought to distance itself from Del Ponte’s remarks, saying in a statement that it “wishes to clarify that it has not reached conclusive findings as to the use of chemical weapons in Syria by any parties to the conflict.”

In the interview, Del Ponte said there were “strong, concrete suspicions but not yet incontrovertible proof of the use of sarin gas” by the rebels. She said those conclusions were based on information about the way the victims were treated.

A U.S. official, speaking on condition of anonymity to discuss the matter candidly, said: “We’re trying to get as many facts as possible to understand when and how such things were used. But our understanding has been that the armed opposition does not have such weapons . . . that they do not have such things in their arsenal.”