President Trump will soon have to withdraw U.S. special forces operators from Syria, according to Russian Foreign Minister Sergey Lavrov.

The top diplomat argued that the U.S. should leave after the impending defeat of the Islamic State in Syria. That would leave Syrian President Bashar Assad, backed by Russian and Iranian forces, in military control of the country. The warning is part of jockeying for long-term control of the strategically significant Syria.

"There is legitimate presence based on an invitation from official authorities, but there is illegitimate one, namely presence of the U.S.-led coalition and special forces' troops from a wide range of foreign countries who nobody has invited," Lavrov told reporters Friday. "After terrorism is defeated, the first step should be the pullout of those who stay illegitimately in Syria."

That statement rests on the assumption that Assad is the "legitimate" leader of Syria, after six years of civil war that saw the Islamic State emerge as a land-holding terrorist group and an international threat. Trump's administration has backed away from former President Barack Obama's declaration that Assad "must go," but the U.S. side still predicts that he would have to leave to bring peace to Syria.

"Nobody in these areas wants the Damascus government to return – meaning flags, army," Brett McGurk, the presidential envoy coordinating the U.S.-led counter-Islamic State coalition, told reporters at the U.N. on Friday. "That would be, I think, something that would not be stabilizing."

McGurk said that Syrian civilians flee areas controlled by the Islamic State or taken from the Islamic State by the Assad regime, whereas they return to their homes in areas that the U.S.-backed Syrian Democratic Forces recapture.

"So, we're finding the pattern for a stable aftermath of ISIS is the Syrian Democratic Forces," McGurk said. "But no, in terms of areas that are retaken [from] ISIS, they will be controlled by the local people who know the areas, pending a longer-term political settlement to the civil war."

Although the threat of the Islamic State dominated short-term debates about Syria, U.S. and Russian leaders have long seen the final outcome of the civil war as having strategic significance for both sides. Assad's regime is a client of Russia that has given the Russians access to a port on the Mediterranean Sea. And Iranian forces are trying to establish a "land bridge" that would allow them to funnel weapons and other material from Iran to their terrorist proxies in Lebanon, on the border of Israel.

"The day after Raqqa falls is going to be the moment that Iran moves to try to oust the United States from the region," Sen. Marco Rubio, R-Fla., told the Washington Examiner in February.

Russian forces have begun targeting the Syrian Democratic Forces, however, in an apparent effort to strengthen Assad's position going into any political process. Lavrov defended the latest strikes by maintaining that the U.S. has been collaborating with anti-Assad terrorists.

"Attempts to make it difficult to finalize the counter-terrorism operation would not be tolerated," Lavrov said.