A coalition of US-backed Kurdish and Arab forces in Syria says it has captured the country's largest oilfield from ISIL fighters.

Laila al-Abdullah, spokesperson for the Syrian Democratic Forces (SDF), said the al-Omar oilfield came under the alliance's control following a "large-scale military operation" in the early hours of Sunday.

The SDF drove ISIL out of the area's fields "with little damage", Abdullah said in a statement, adding that its forces were now clashing with ISIL fighters holed up in an adjacent housing complex.

The coalition is in a race against Russian-backed Syrian governments troops to seize parts of the resource-rich Deir Az Zor province bordering Iraq.

Al-Omar is located on the east bank of the Euphrates River in eastern Deir Az Zor. Forces loyal to Syrian President Bashar al-Assad have also gained ground in recent weeks on the west bank of the river, advancing under the cover of Russian air raids.

'Delicate situation'

The Syrian Observatory for Human Rights, a UK-based monitor tracking developments in Syria's conflict via a network of sources on the ground, said on Sunday it had been informed that the SDF had "managed to enter" the oilfield.

It said the SDF's advance in al-Omar came after government forces withdrew from the area following an attack by the Islamic State of Iraq and the Levant (ISIL, known as ISIS) group fighters.

Al Jazeera's Hashem Ahelbarra, reporting from Gaziantep, on the Turkish side of the Syria-Turkey border, said the capture of al-Omar meant that "the SDF is now going to have vital financial resources to continue its fight and keep expanding."

"The Syrian army is just about a few kilometres away from the same oilfield that was captured on Sunday," he said.

"So we're talking about still a very delicate situation facing the SDF as they are trying to capture more territory from ISIL."

Last month, the SDF said it had captured the Conoco gas field and plant in Deir Az Zor.

Earlier this week, the US-backed fighters announced the capture of Raqqa, ISIL's self-proclaimed capital in Syria, after a four-month operation.