US Secretary of State John Kerry said on Friday it might be possible for the Syrian government and rebel forces to cooperate against ISIS militants without Syrian President Bashar al-Assad having first left power.
However, Kerry said it would be "exceedingly difficult" to achieve this if rebel forces that have been fighting against Assad for more than four years did not have some confidence that the Syrian leader would eventually go.
Kerry was asked at a news conference during a visit to Greece whether Assad's departure was a precondition for Western-backed rebels to cooperate with government troops against ISIS, which has captured a swathe of Syria and Iraq and carried out a string of attacks in other countries.
"With respect to the question of Assad and the timing, I think the answer is ... it is not clear that he would have to 'go' if there was clarity with respect to what his future might or might not be," Kerry said.
That clarification could come in many forms that would give certainty to the opposition.
"But it would be exceedingly difficult to cooperate without some indication or confidence on the part of those who have been fighting him that in fact there is a resolution or a solution in sight," Kerry added.
Otherwise the rebels would feel they were helping and entrenching Assad, which would be completely unacceptable, he said.
Russia and Iran, Assad's main allies, have said it will be up to the Syrian people to decide on Assad's role at a future presidential election.
Russia has intervened militarily in support of Assad with air strikes against both ISIS and Western-backed rebels, while a US-led coalition of Western and Sunni Arab states has been waging an air campaign against ISIS in Syria and Iraq.
A US official who spoke to reporters on condition of anonymity said Kerry's message was that Assad "doesn't have to go right now", provided there was a clear political transition in prospect, a position Washington has held for months.