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) in the enclave, AFP reported.
"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 IS."
Major Adrian Rankine-Galloway, also a Pentagon spokesman, said the U.S. 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."
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