BEIRUT (Reuters) - The Syrian Kurdish YPG militia said on Sunday it had fully captured Deir al-Zor’s eastern countryside from Islamic State with the help of both the U.S.-led coalition and Russia.

The YPG spearheads the Syrian Democratic Forces (SDF), an alliance of mostly Kurdish and Arab militias fighting Islamic State with Washington’s backing. The SDF offensive in Deir al-Zor province, which borders Iraq, has focused on territory east of the Euphrates river.

On the western bank, the Syrian military has waged its own attack against Islamic State with support from Iran-backed militias and Russia.

The separate assaults advanced from opposite sides of the river, which bisects oil-rich Deir al-Zor, mostly staying out of each other’s way. The U.S.-led coalition and the Russian military have held deconfliction meetings - to prevent clashes between planes and troops - though the two offensives have sometimes come into conflict.

The SDF’s most powerful component, the YPG, announced that it had defeated Islamic State militants on its side “in cooperation with the Arab tribes.”

The U.S.-led coalition and Russian forces in Syria provided “air and logistical support, advice and coordination on the ground,” its statement said. “We hope for an increase in this support and for ensuring the necessary air cover.”

A YPG spokesman declared the victory in a village in Deir al-Zor in the presence of a Russian envoy from Moscow’s Hmeimim military base in Syria, it said.

U.S. Defense Secretary Jim Mattis said this week that as the battle against Islamic State entered its final stages, he expected the focus to move toward holding territory instead of arming Syrian Kurdish fighters.

Washington’s support for the YPG has infuriated Turkey, which sees the growing influence of Kurdish forces on its border as a security threat. Ankara considers the YPG an extension of the outlawed Kurdistan Workers Party (PKK) movement, which has fought a decades-long insurgency on Turkish soil.

Since the start of Syria’s conflict in 2011, the YPG and its allies have carved out autonomous regions in the north. They now control around a quarter of Syria, the largest part outside government control, after seizing vast land from Islamic State.

Their autonomy plans face opposition from their battlefield ally Washington, from Turkey and from the Syrian government.

The YPG said on Sunday it was forming civil councils for Deir al-Zor that include Kurds, Arabs and other components based on “the principle of democracy and self-rule.” It would mirror governing arrangements in other towns and cities that the Kurdish fighters and their allies captured.

With Islamic State close to collapse in Syria, Syrian Kurdish leaders hope for a phase of negotiations to shore up their autonomy in the north. But in recent months, the Damascus government and its Iranian allies have more forcefully asserted ambitions to take territory under the control of Kurdish-led forces.