US Secretary of Defence Jim Mattis warned that coalition forces leaving Syria could create a "vacuum"

US Secretary of Defence Jim Mattis warned Friday it would be a "strategic blunder" to pull out of Syria before UN-led peace efforts had made progress.

A US-led coalition is conducting military operations against the Islamic State group in Syria and Mattis said they must not leave a "vacuum" that President Bashar al-Assad and his Russian and Iranian allies could take advantage of.

Talks in Geneva led by UN Special Envoy Staffan de Mistura have made little headway, but Mattis said they must be given the chance to succeed.

"In Syria, leaving the field before the special envoy Staffan de Mistura achieves success in advancing the Geneva political process we all signed for under the UN security council resolution would be a strategic blunder, undercutting our diplomats and giving the terrorists the opportunity to recover," Mattis said at a meeting of coalition defence ministers at NATO headquarters in Brussels.

IS seized parts of a town on the Syria-Iraq border on Friday in the latest in a string of attacks that comes as the continued presence of coalition forces in Syria is coming into question.

US President Donald Trump has vowed he would pull out his troops from Syria but Mattis has pleaded for a more patient approach.

"As the operations ultimately draw to a close, we must avoid leaving a vacuum in Syria that can be exploited by the Assad regime or its supporters," Mattis said.