US-backed SDF captures one of Syria's largest gas fields: Commander

US-backed forces in Syria have seized a major natural gas field in Deir Ezzor province from the Islamic State group (IS) after days of fighting, a group commander said.

Ahmed Abu Khawla told Reuters on Saturday that the Conoco gas field was the first of its kind taken by the Syrian Democratic Forces (SDF) - an alliance of Kurdish and Arab militias backed by the US - since it began an offensive earlier this month to capture the eastern province.

But the Syrian Observatory for Human Rights, which tracks the war in Syria, says SDF fighters have not yet taken the Conoco gas field, saying fighting is ongoing outside it.

Deir Ezzor is one of the few remaining Syrian towns still under IS control. The group is battling two separate offensives, as the SDF pushes on the town from the eastern bank of the Euphrates river and the Syrian army and its allies approach from the western bank.

Syrian troops supported by Iranian-backed militias have also crossed to the eastern side of the river, which runs through the city of Deir Ezzor, increasing their presence in an area where US-backed fighters have also advanced.

Abu Khawla said the Syrian army and its allies were within four kilometres of SDF positions.

Deir Ezzor province is rich in oil and gas and both sides have been racing to reach the fields. The next main target will be al-Omar oil field, which is Syria's largest and is also on the east bank of the Euphrates, and Syrian government forces are also speeding to capture it. Oil revenues are badly needed for future reconstruction of Syria that has been plagued by war since 2011.

Tensions as Russian and US-backed forces meet

Both the United States and Russia have been carrying out air strikes in support of groups on the ground and both nations have deployed special forces.

Russia said on Thursday it had warned the United States it would target areas in Syria where US Special Forces and the SDF were operating if its own forces came under fire from them, something it said had already happened twice.

Last week the US-backed militia said six of its fighters were wounded in an air strike near Deir Ezzor. The US military said in a statement Russia forces had been behind the strike, something Moscow denied.

"Russian air forces carry out pinpoint strikes only on IS targets that have been observed and confirmed through several channels," said the Russian defence ministry spokesman, Major-General Igor Konashenkov, according to Associated Press.

The Russian warning underscored growing tensions over Syria between Moscow and Washington. While both oppose Islamic State, they are engaged, via proxies, in a race for strategic influence and potential resources such as the oilfields in Deir Ezzor province.