Syrian government troops came closer Monday to encircling the Islamic State group in a pocket of Deir Ezzor city after crossing the adjacent Euphrates River, Moscow and a monitoring group said.

Russian-backed Syrian forces are trying to tighten the noose on jihadists still inside the city on the river's western bank.

The army has sealed off the city on three sides, but IS still controls eastern districts along the river, which both jihadists and civilians had used as an escape route.

On Monday, elite Syrian troops crossed the river, Russia's defence ministry said.

"Today, 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 Ezzor region," a ministry statement said.

It said "shock troops" had already captured several villages on the river's eastern bank from IS and were pushing further east.

The Britain-based Syrian Observatory for Human Rights told AFP that Syrian commandos and reconnaissance units had crossed the river using a floating bridge.

"This paves the way for completely besieging the city," said Observatory head Rami Abdel Rahman.

The Euphrates slices diagonally across Deir Ezzor province, an oil-rich eastern region bordering Iraq.

Until Monday, Syrian troops had been fighting only west of the Euphrates, while the US-backed Syrian Democratic Forces waged a rival offensive against IS east of the river.

The SDF has captured more than 500 square kilometres (190 square miles) in northeastern parts of the province, according to the US-led coalition which is providing air cover.

To prevent the two operations from clashing, the coalition, the SDF, Syria's government and Russia have agreed on a "de-confliction line" in northeast Syria.

That line runs from the neighbouring province of Raqa southeast along the Euphrates to Deir Ezzor.

Coalition spokesman Colonel Ryan Dillon declined to say whether the Syrian army crossing the river violated the de-confliction line.

"The closer together the Syrian army and the SDF get, the more awareness is going to be required," Dillon said.

The arrangement was tested on Saturday after the US-led coalition and the SDF accused Russian warplanes of bombing SDF fighters east of the Euphrates, a claim Moscow denied.

General Joe Dunford, chairman of the US Joint Chiefs of Staff, said he spoke to his Russian counterpart General Valery Gerasimov on Sunday.

"We have been engaged at every level to re-establish de-confliction at the Euphrates river," he said.

Russian jets were pursuing IS fighters who had fled across the Euphrates when their jets struck close enough to injure nearby SDF troops, according to Dunford.

The skies over Syria have become increasingly congested as the six-year conflict drags on, with warplanes from the coalition, the government and Russia all carrying out strikes.

More than 330,000 people have been killed and millions displaced since Syria's conflict broke out in 2011.

Source: AFP