Kurdish members of the Syrian Democratic Forces -- made up of Kurds, Syrian Arabs and other groups -- had quit the Middle Euphrates River Valley in February after Turkey sent troops into Syria and launched an operation against the Kurdish People's Protection Units (YPG) militia in its Afrin enclave.

Turkey drove the group from the city on March 18.

The sudden departure of SDF fighters resulted in a slow-down in missions against IS in two remaining jihadist pockets along the river.

"We are encouraged by the return of some Syrian Democratic Force partners to the Middle Euphrates River Valley," said Colonel Ryan Dillon, a spokesman for the US-led coalition fighting IS in Iraq and Syria.

"More combat power is returning to Euphrates River Valley to really turn it on to the ISIS elements," he added.

Dillon did not provide any details on the numbers of fighters returning.

The United States has since late 2014 led an international coalition to defeat IS in Syria and Iraq.

President Donald Trump recently threw America's commitment to Syria into doubt, especially after IS is defeated, when he said he wanted US forces out as soon as possible.