DAMASCUS (AFP) - Syrian troops broke the Islamic State in Iraq and Syria (ISIS) group's siege of Deir Ezzor military airport on Saturday (Sept 9), dealing a fresh blow to the militants who also face a new offensive from US-backed fighters elsewhere in the province.

Oil-rich Deir Ezzor province borders Iraq and is a strategic prize for both the US-backed Syrian Democratic Forces (SDF) and Russian-supported government troops.

Since 2014, ISIS has held swathes of the province and about 60 per cent of its provincial capital, encircling two regime-held enclaves in the western half of Deir Ezzor city.

Government troops broke the siege of one of the pockets on Tuesday, and began a new assault on ISIS positions around the strategic airbase on Friday.

On Saturday, the troops ended ISIS' encirclement of the airport, state news agency Sana said, "after forces advancing from the cemetery southwest of the city linked up with the forces holding the airbase".

State television quoted the unnamed head of the airbase as saying IS had fiercely attacked the airport for years. "We will continue fighting until we recapture all of Deir Ezzor city," he said.

Russia, a key Syrian government ally, said its warplanes had provided cover to the Syrian ground forces, carrying out "massive air strikes" that helped the troops break the airport siege.

"Syrian government forces, with Russian air support... inflicted a crushing defeat on ISIS around Deir Ezzor surpassing all the other victories achieved in the past three years," the ministry said.

The Britain-based Syrian Observatory for Human Rights said the advance enabled Syrian troops to "link up all the neighbourhoods they hold in western parts of Deir Ezzor city".

The government's progress in Deir Ezzor came as ISIS faces growing pressure in both Syria and Iraq, where it has suffered a string of defeats.

'OPERATION JAZIRA STORM'

The militants have lost Iraq's second city, Mosul, and more than half of their de facto Syrian capital Raqa.

Deir Ezzor province is the last one in Syria still largely under militant control.

On Saturday, the SDF said it had begun clearing ISIS from areas east of the Euphrates River, which cuts diagonally across Deir Ezzor province, slicing it in two.

"We are taking the first step to liberate territory east of the Euphrates River in Deir Ezzor," Ahmad Abu Khawlah, who heads the SDF's Deir Ezzor Military Council (DEMC), told AFP.

"Pivoting towards Deir Ezzor was inevitable."

Abu Khawlah said US-led coalition air strikes had already helped his forces advance "around 30km" in the first day of "Operation Jazira Storm".

The commander of the US-led coalition, Lieutenant-General Paul Funk, welcomed the start of the SDF operation.

"The very existence of ISIS poses a real threat to the civilised world and our way of life," Funk said in a statement.

"Our collective effort will defeat them."

Military operations were focused on the hilly desert area in northeastern parts of Deir Ezzor province, the Observatory said.

'NO SAFE HAVEN'

The coalition has bombed ISIS targets in Syria and Iraq since 2014 and is now backing the SDF's offensive to capture Raqa.

The SDF has seized around 65 per cent of Raqa city, which lies directly west of Deir Ezzor province.

"Daesh will have no safe haven in the Euphrates River Valley," coalition spokesman Colonel Ryan Dillon said earlier, using the Arabic expression for ISIS.

Abu Khawlah said SDF forces were not coordinating with the government or with Russia on military operations.

But the coalition, the SDF, Syria's government and Russia have agreed on a "de-confliction line" in northeast Syria to prevent them clashing.

According to the coalition, the line runs from Raqa province southeast along the Euphrates towards Deir Ezzor.

"The de-confliction line with the Russians is necessary in the complex and congested battlespace over eastern Syria," Dillon said.

Syria's conflict erupted in 2011 with protests demanding that President Bashar al-Assad step down, and has since evolved into a complex, multi-front war.

More than 330,000 people have been killed and millions have fled their homes, but global efforts at reaching a peace deal have faltered.

Next week, Kazakhstan will host talks on a set of safe zones across Syria agreed by world powers this year as a first step towards a political solution.

And the United Nations is hoping to gather members of the Syrian opposition and the government for negotiations in Geneva in October.