The Donald Trump administration’s decision to keep a residual force in Syria has temporarily reassured America’s Kurdish allies, who have publicly floated the possibility of looking to Bashar al-Assad for protection. But officials fear the US-backed force fighting the Islamic State (IS, also known as ISIS) could still fracture amid uncertainty over US intentions.

White House spokeswoman Sarah Sanders said Thursday that the United States would keep a peacekeeping force of 200 troops in Syria “for a period of time,” further muddling the president’s timeline for withdrawal. Administration officials upped that number to 400 today, and the Pentagon said the US units would be split evenly between al-Tanf, a training facility on the border with Iraq, and northeastern Syria to protect the Kurdish-dominated Syrian Democratic Forces (SDF) from Turkish attack.

“US forces numbering a couple of hundred will remain in northeast Syria as part of a multinational force,” said Pentagon spokesman Sean Robertson. “Separately, the US will maintain a presence at the al-Tanf garrison in southern Syria. Multinational observing and monitoring force will be made of primarily NATO allies who, along with US forces, will maintain stability and prevent an ISIS resurgence in Syria.”

The presence at al-Tanf, which straddles an Iranian supply line into Syria, will be only a US presence, Robertson said, adding that the US withdrawal “will continue in a deliberate and coordinated manner.”