Biden: US would hold direct talks if Iran serious

"This is an important step, given that the coalition was created on the platform of rejecting talks with the regime," he said. "But I think that realism will prevail. Naturally, this does not guarantee that dialogue will start, as the opposition does not have a negotiating team and there are many different groups to agree on a single delegation."

At the conference, Biden told an audience including Lavrov that despite differences, "we can all agree on the increasingly deep plight of the Syrian people and the responsibility of the international community to address that plight."

But Lavrov fired back that "there are a lot of question marks about the Western approaches to those developments," in the region, and questioning when it is "permissible to cooperate with regimes and when is it legitimate to argue for their removal."

Lavrov also suggested Biden's statement that Assad must go was counterproductive.

"The persistence of those who say that priority No. 1 is the removal of President Assad — I think it's the single biggest reason for the continued tragedy in Syria."

Asked whether Russia might be prepared to endorse humanitarian corridors protected by air power, Lavrov bluntly replied: "No. Any threat of use of force will be unacceptable."

Jim Heintz contributed from Moscow.