“The Syrian Kurdish Y.P.G. do not truly desire to shed blood to capture a majority-Sunni Arab city far from their vision of their autonomous borders, while Turkey cares about the operation only insofar as the Syrian Kurdish Y.P.G. is not allowed to participate in it,” said Christopher Kozak, a Syria researcher at the Institute for the Study of War.

Defense Secretary Ashton B. Carter said in Brussels on Wednesday that the offensive to oust the Islamic State, also known as ISIS or ISIL, from Raqqa would begin within weeks.

“This is, as always, a matter when you’re positioning forces and so forth, we have a plan to do that and a schedule to do that,” Mr. Carter told reporters. “We’re going to execute to that plan.”

Mr. Carter met with the Turkish defense minister, Fikri Isik, and the French defense minister, Jean-Yves Le Drian, on Wednesday in Brussels, where he emphasized the “need for the coalition to maintain pressure on ISIL on multiple fronts,” according to a Pentagon news release. “All sides agreed to maintain frequent communication on the full range of security interests, and to continue their close coordination and continued transparency in the coalition effort to deal ISIL a lasting defeat,” the statement said.

An American military official said the Raqqa operation would take place in roughly three phases.

Phase one, he said, is what the American-led coalition fighting the Islamic State has been doing for months: preparatory airstrikes in and around Raqqa to knock out command-and-control and fighting positions. Phase two, to begin in the coming weeks, will be to isolate Raqqa with the available forces — mostly Syrian Kurds, with Syrian Arabs, too. Phase three will be the fight for Raqqa itself, which American officials say they hope will be conducted mostly by Syrian Arabs, given that the city is majority Sunni Arab.

General Townsend compared Islamic State plotting in Raqqa to planning by the group in Manbij, which was retaken from the Islamic State in August.

Manbij was the last stop on the route out of Syria for Islamic State militants headed to Europe. But the actual plotting regularly began in Raqqa. Militants moved from there to Manbij before slipping over the border into Turkey and then onward to Europe.