BEIRUT (Reuters) - Syrian troops seized a suburb of the eastern city of Deir al-Zor on Sunday, tightening the noose around Islamic State, a Syrian military source said.

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The army pushed into the city this month with the help of Russian air power and Iran-backed militias, breaking an Islamic State siege of an enclave there that had lasted three years.

On Sunday, the Syrian army and allied forces captured al-Jafra district on the western bank of the Euphrates river, the military source said.

“They have no outlet except crossing the Euphrates towards the eastern bank and fleeing towards the desert, or (the towns) al-Bukamal and al-Mayadin,” the source told Reuters.

Moscow and Washington are backing separate offensives in the oil-rich province of Deir al-Zor bordering Iraq. Both have advanced from opposite sides of the Euphrates which bisects the province, Islamic State’s last major foothold in Syria.

Russian- and U.S.-backed offensives against Islamic State have mostly stayed out of each other’s way, with the Euphrates often acting as the dividing line.

But the Pentagon accused Russia this week of bombing U.S.-backed forces on the river’s eastern bank.

Russia’s defence ministry rejected the allegations on Sunday. Moscow had warned the United States well in advance of its operational plans and its jets only targeted Islamic State militants, it said.

RIVER ESCAPE

Russia’s RIA news agency cited an unnamed source as saying the Syrian army had cut Islamic State’s main supply line in Deir al-Zor city.

The British-based Syrian Observatory for Human Rights said government forces took al-Jafra near the city’s air base overnight, though Islamic State militants still hold nearly a third of the city.

Russian jets pounded movements across the river as Islamic State fighters tried to escape in ferries, and many civilians, including families of the militants, had also tried to flee across the river in recent days, it said.

Separate air strikes by Russia and by the U.S.-led coalition killed more than 34 people, including children, across Deir al-Zor province over the past day, the war monitor said.

Islamic State controls much of the desert region around Deir al-Zor city, where its fighters are also under attack from the U.S.-backed Syrian Democratic Forces (SDF) alliance.

With jets and special forces from the U.S.-led coalition, the alliance of Kurdish and Arab militias is battling Islamic State in the northern parts of Deir al-Zor province.

The SDF said it had taken 14 villages and farms, two towns, and some factories on the eastern bank of the Euphrates since launching its assault last week.