President Donald Trump last week said he wants the US presence in Syria to end "very soon." On Wednesday the White House vowed the US mission would come to a "rapid end," without actually setting a timeline.

A key issue is the fate of the thousands of Syrian Kurds that for years have been fighting the Islamic State group with US training, equipment and air support.

"We are in consultation with our allies and partners right now, so we'll work all this out," Mattis said when asked whether the US military is committed to supporting Syria's Kurdish fighters.

Many Kurds have already quit fighting the Islamic State group in Syria's eastern Deir Ezzor province so they can instead fight a Turkish offensive around Afrin in the north.

It is unclear what would happen to the Kurdish fighters after a US pull out, including whether they would be drawn into a broader fight with NATO member Turkey.