A powerful Syrian Kurdish militia announced on Sunday it had cleared Islamic State group fighters from key territory east of the winding Euphrates River.

In a surprising announcement, the People's Protection Units (YPG) also said the victory in Syria's Deir Ezzor province came with logistical and air support from Russia, as well as the US-led coalition, its traditional backer.The YPG was spearheading a US-backed offensive to oust IS from territory east of the Euphrates River, which cuts diagonally across Deir Ezzor, while rival Russian-backed Syrian troops battled IS on the western bank.

It appears to the first time Syria's Kurds acknowledge receiving direct Russian support.

On Sunday, YPG officials met with a Russian military delegation in a town on the eastern bank of Deir Ezzor province, according to the Kurdish-run ANHA news agency.

"The eastern countryside of the city has been fully liberated from Islamic State... with the international coalition and Russia's support," YPG spokesman Nuri Mahmoud said in a statement.

"We thank the international coalition and the Russian forces for their air, logistical, advisory support and coordination on the ground."

Mahmoud said the YPG "hoped for an increase in support and the provision of air defence and required cover."

IS jihadists once held most of oil-rich Deir Ezzor, but now control only a tiny sliver of the province, according to the Britain-based Syrian Observatory for Human Rights monitor.

On Sunday, Observatory head Rami Abdel Rahman said IS still held several villages in the province, on both banks of the Euphrates River.

The YPG formed the bulk of the Syrian Democratic Forces, an Arab-Kurdish alliance that ousted IS from swathes of northern and eastern Syria, with help from the US-led coalition.

In September, the SDF began its assault on IS positions in the eastern half of Deir Ezzor province, as Russian-backed government troops squeezed IS in the western half.

The SDF accused Moscow of striking its forces several times during the drive, which makes Sunday's announcement of Russian support all the more surprising.

Last week, the White House said it would be making "adjustments" to its military support of militia in Syria.

The US-led coalition has provided air support, weapons, and advice to the YPG and SDF in their push against IS.