WASHINGTON — Iran could join international talks over Syria’s future for the first time later this week, as the United States declared itself ready to engage its long-time foe if it might help halt Syria’s four-year civil war.

Washington was still waiting to hear if Tehran will attend the next round of discussions, expected to start Thursday and continue Friday in Vienna. Secretary of State John Kerry, Russian Foreign Minister Sergey Lavrov and several top European and Arab diplomats will be attending, a familiar cast that up to now hasn’t included any Iranian representatives. Kerry departs for the Austrian capital on Wednesday.

State Department spokesman John Kirby told reporters on Tuesday the arrangements for Vienna were still being finalized, but that “we anticipate that Iran will be invited to attend this upcoming meeting.” U.S. officials told The Associated Press that Russia was reaching out to Iran; Lavrov has spoken to Foreign Minister Mohammad Javad Zarif multiple times in recent days, according to the Russian foreign ministry.

While the U.S. doesn’t approve of Iran’s “destabilizing activities” in Syria, Kirby said American officials “always have recognized that at some point in the discussion, moving toward a political transition, we have to have a conversation and a dialogue with Iran.”

“It’s up to Iran to decide whether they’re going to or not when they are asked,” he said.

The United States is taking a gamble. Iran has backed President Bashar Assad’s government throughout the conflict, fighting alongside the Syrian military, and is seen by Western-backed rebels and U.S. partners in the region as a major source of the bloodshed. The Syrian opposition may balk at Iran’s inclusion in any discussions on what a post- Assad Syria should look like.

On the other hand, all previous international efforts have done nothing to stop the fighting, and Kerry is trying to unite all sides with influence in the Arab country around a common vision of a peaceful, secular and pluralistic Syria governed with the consent of its people.

Washington actively opposed Iran participating in two earlier, months-long mediation attempts but recently spoke of the possibility of Iran joining talks in the future. It is now offering Tehran a seat after days of behind-the-scenes negotiation, particularly with its regional rival Saudi Arabia, according to officials who weren’t authorized to speak publicly on the matter and demanded anonymity

In a telephone conversation Tuesday, President Barack Obama and Saudi King Salman spoke about cooperating closely to fight the Islamic State and “establish the conditions for a political transition in Syria,” according to a White House statement. They vowed to build on recent diplomatic efforts. The statement didn’t mention anything about Iran.

It’s not a given that Iran attends. Its supreme leader, Ayatollah Ali Khamenei, ruled out new negotiations with the United States after they and five other nations clinched a long-term nuclear agreement in July. But Iran clearly has a stake in Syria’s future, as Assad’s government has helped the Iranians exert dominance over nearby Lebanon and threaten Israel through their proxy, Hezbollah.

The U.S., Russia, Saudi Arabia and Turkey met last week in Vienna, putting forward new ideas to revive diplomatic hopes. However, they remained deeply divided over Assad’s future.