Syrian army and allies close in on Islamic State in Deir al-Zor

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 US-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. 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.