Syrian troops battling ISIL crossed to the eastern bank of the Euphrates River in Deir Az Zor on Monday, 
The Syrian army sealed off Deir Az Zor on three sides as of Monday, but ISIL still controls eastern districts along the river, which both the group and civilians had used as an escape route.
On Monday, elite Syrian forces crossed the river, Russia's defence ministry said, Syrian government forces - reinforced by a unit of the 4th Armoured Division and with the support of Russian aviation - crossed the Euphrates River in the Deir Az Zor region," a ministry statement said.
It said "shock troops" had already captured several villages on the river's eastern bank from ISIL and were pushing further east. 
A commander in a militia of the US-backed Syrian Democratic Forces (SDF) confirmed Syrian army units had crossed and said his fighters were ready to drive them back.
"If there are clashes between us and them - we're ready for those if the forces of the regime don't go back to the other bank," Ahmed Abu Khawla of the SDF's Deir Az Zor military council said.
Rami Abdurrahman, head of the UK-based Syrian Observatory for Human Rights war monitor, said the advance across the river was preceded by intense air raids on the eastern bank.
"Even if the pro-government [forces] keep up their advance in the city, it will mean nothing if they don't control the eastern bank," Abdurrahman said.