The U.S. is ready, however, to work with any nation – including Russia and Iran – to resolve the conflict, spokesman Eric Schultz told reporters ahead of Syria talks set to begin Thursday in Vienna.

Russia and Iran has backed the Syrian regime of President Bashar al-Assad since the war began in 2011. But the U.S. seeks a political transition without Assad.

Although Iran is not considered as a key partner in the discussions, the U.S. wants Tehran to take place in the talks as a “stakeholder”.

“The Secretary of State [John Kerry] has made clear that Iran is going to have to be part of this discussion and our bottom line is we believe it’s important for all key stakeholders to be present,” Schultz said.

A State Department spokesman said a political transition was a “complicated process”.

“We expect it’s going to continue to take some time and I certainly would expect that while there continues to be momentum, we’re not at a consensus yet,” said John Kirby.

Foreign ministers of Turkey, U.S., Russia and Saudi Arabia participated in the last meeting on Syria’s future held last week in Vienna.

Egypt, Qatar, Lebanon, the EU represented by France, and Iran, are also expected to participate in the next talks.

Iran had been invited to the talks for the first time and had agreed to participate.

The U.S. has considered Iran participation in the talks for some time but Saudi Arabia had rejected the idea.