US-led coalition fighter jets reportedly bombarded pro-Syrian government forces on Sunday, rolling back their advance into oil-rich areas held by the Syrian Democratic Forces in eastern Syria.

At least nine pro-government fighters were killed in the operation, the Syrian Observatory for Human Rights activist group said, hours after the Syrian army claimed it had "liberated" four villages in Deir Ezzor province from the US-backed SDF.

If confirmed, the attack would be a significant departure from the international coalition's stated mission of battling the Islamic State group (IS) in Syria, instead seeing the US-led force fighting over territory not currently contested by the group.

Coalition aircraft... inflicted heavy losses - SDF spokesman

In February, US air strikes killed hundreds of pro-government fighters, including Russian paramilitary contractors, in the same oil-rich area near the Iraq border.

A Western diplomatic source earlier told Reuters that coalition jets from bases in northern Syria hit the attacking forces, which were believed to include Iranian-backed militias operating in the Deir Ezzor area.

A source in the Kurdish-led SDF also confirmed coalition forces had intervened, and local news outlet Euphrates Post reported "coalition aircraft... inflicted heavy losses" on pro-Syrian government forces.

The US army confirmed the offensive on the SDF in a statement but would not elaborate on its role in the counter-operation, mentioning only that it had used "used established deconfliction channels to de-escalate the situation."

SDF fighters patrol Raqqa during liberation from Islamic State (Reuters)

SDF spokesman Kino Gabriel said his forces had driven back the "the regime's army and militias supported by the Russian side", without any mention of the jets' role.

Ilham Ahmed, co-chair of the SDF's political wing, accused Iran, Russia and Turkey of conspiring to support the attack on its forces in Deir Ezzor, reported Kurdish news agency Rudaw.

The SDF, a US-backed force lead by the Kurdish YPG militia, last year took control of large areas east of the Euphrates river in Deir Ezzor province as part of a major aerial and ground campaign led by the Pentagon that drove IS militants from much of eastern Syria and their former de-facto capital Raqqa.

The Russian-backed Syrian army has mostly avoided conflict with the SDF, instead focusing its efforts in eastern Syria on regaining territory from IS militants west of the river.

There have been elaborate "deconfliction lines" separating the US-backed forces on the eastern side of the river from the Russian and Iranian-backed forces on the western side to prevent clashes, US army officials and defence analysts say.

Before the announcement that the Syrian army had taken control of the villages, the SDF said they were engaged in heavy clashes with Syrian army troops on the outskirts of the village of Janin near the Euphrates.

The SDF accused Syrian government authorities of seeking to disrupt the US-led coalition's preparations to resume an imminent offensive against IS militants in several pockets of territory that they still control in the Euphrates River Valley.

US Defence Secretary Jim Mattis said on Thursday he expected a "re-energised" effort against IS militants in eastern Syria in the coming days.

US officials have said that in recent days they have seen SDF fighters returning to the areas around the Euphrates where the militants operate to relaunch new operations against them.

The YPG has blamed a Turkish offensive against it in northern Syria's Afrin for diverting efforts away from the fight against IS.