The SDF were the main ground partner in the US-led campaign against the Islamic State group, and US President Donald Trump has faced a firestorm of criticism for abandoning a loyal ally.

"There'll be ongoing conversations on what kind of capabilities we can help bring to them to continue the fight within Syria," the Pentagon official said, requesting anonymity.

"We continue to be committed to the de-ISIS campaign and we want to figure out how we can continue working with the SDF.

"They have a very strong robust relationship with the US military... and we think we can preserve that relationship."

The US is to withdraw more than 1,000 troops from northern Syria, keeping only a residual contingent of around 150 at the Al-Tanf base near the southeastern borders with Jordan and Iraq.

Trump announced the pullback last week, a move widely interpreted as green-lighting a long-planned Turkish invasion of northern Syria.

As Turkish forces advanced, Syria on Tuesday dispatched more forces to beat back the offensive, while Russia deployed patrols to prevent clashes between the two sides.

European governments are worried the chaos could trigger mass breakouts by thousands of IS fighters detained by Kurdish forces, and a broader resurgence of the jihadist movement.

The SDF lost 11,000 fighters in the campaign against IS before finally overrunning the group's self-proclaimed "caliphate" in March.