The Syrian Democratic Alliance, made up mainly of Kurds as well as Syrian Arabs, is the main ground force that the US has been training and supporting to battle IS.

While most of the land once held by the jihadists has been recaptured, the extremists remain entrenched in the Middle Euphrates River Valley (MERV).

Some SDF in the area have now gone to Afrin, where Turkish troops and allied rebels have for six weeks been attacking Kurdish People's Protection Units (YPG) militia in the enclave.

"Operational pauses occur regularly for a variety of reasons," Pentagon spokesman Colonel Rob Manning said.

"The nature of our mission in Syria has not changed. ... This operational pause will not cause us to lose sight on our main objective, which is ISIS."

Major Adrian Rankine-Galloway, also a Pentagon spokesman, said the US was continuing its own operations against IS, and none of the territory recaptured from IS had been lost.

"We are aware of the departure of some SDF forces from the Middle Euphrates River Valley and continue to point out the potential costs of any distraction from the defeat-ISIS fight," he said, adding that the SDF was "continuing to contain and degrade ISIS in the Middle Euphrates River Valley."

The acknowledgement that some SDF fighters are peeling away from the fight in the MERV comes after the head of Central Command, General Joe Votel, warned lawmakers of "diverging interests" in the region.

"Our concern, of course, is that this activity in Afrin is detracting from our efforts against ISIS," Votel told the House Armed Services Committee last week.