WASHINGTON - The U.S.-led coalition fight against Islamic State in Syria is not over, even though the group no longer controls territory, the chief U.S. diplomat for Syria warned on Monday and said the whereabouts of its leader Abu Bakr al-Baghdadi were unknown.

U.S. envoy for Syria, Jim Jeffrey, speaking to a briefing at the State Department, said Islamic State's loss of its last territorial stronghold in Syria over the weekend was a 'great day' but the fight against the group will go on and U.S. forces would remain in Syria in limited numbers to help.

'This is not the end of the fight against ISIS. That will go on, but it will be a different kind of fight,' Jeffrey told reporters. 'ISIS has lost much of its capability to project terrorist power and to have a recruiting base in an area that it controls. So it's a very, very important development,' he added.

Jeffrey said the focus of the coalition was on the enduring defeat of Islamic State and pressing governments to take back foreign fighters captured during the conflict.

He said most of the prisoners captured by the U.S.-based Syrian Democratic Forces (SDF) were Iraqis and Syrians. Those, he said, would be returned to their communities for 'deradicalization,' reintegration, and in some cases, punishment.

He dismissed reports that the U.S.-coalition was considering an international tribunal to try the prisoners. 'We're not looking at that right now,' he said.

It was difficult to estimate how many prisoners, including Iraqis, Syrian and foreign fighters, were being held by the SDF, he said.

'People are reporting to me we're in the 7,000 or more (range) but it may have gone up over the weekend,' Jeffrey said.

Ahead of the final assault on Baghouz at the weekend, the SDF said it had 800 foreign fighters and more than 2,000 Islamic State wives and children.