The move could potentially put the American troops in the cross hairs of Russia, which last month began airstrikes on antigovernment forces in Syria. A senior Pentagon official said the United States had not informed Moscow about where the American forces will be. “The area where we are planning to place these special operators is not an area where they have struck or where they would need to strike,” the official said. “It’s not ISIL or regime-controlled.”

But given that most Russian airstrikes have been against opposition groups that are not part of the Islamic State, there is no guarantee that Moscow may not hit groups in which the American forces are embedded. That said, Russia has “significant visibility” over what happens in Syria, the official said, adding that if it becomes necessary to keep its troops safe the United States will communicate with Moscow.

But Mr. Miller and other foreign policy specialists warned that the move risked alienating Turkey since it will bring American forces into an even closer alliance with northern Syria’s Kurdish fighters, including Kurdish People’s Protection Units, which have come under attack from Turkey in recent days.

Derek Chollet, a former assistant defense secretary under Mr. Obama, said the administration would seek to calm Turkish concern by saying the move would help protect its interests. “After all, the Turks have wanted U.S. Special Forces on the ground in Syria for years,” he said.

The diplomatic progress reported in Vienna came after seven hours of heated negotiations, punctuated by sharp exchanges between the foreign ministers of Iran and Saudi Arabia. But it amounted to more of an aspiration than a settlement.

Mr. Kerry and the other diplomats set no deadline for either the cease-fire or a new constitution and election that would follow, and it remained unclear whether Mr. Assad, who was not invited, or the rebels seeking to overthrow him will agree. But the diplomats will meet again in two weeks to expand on the basic principles issued on Friday night, including a commitment to keep Syria together as a single nation.