Syrian state media reported the overnight air raid in an area where both sides have been battling holdout jihadists, but said it only caused material damage.

The Britain-based Syrian Observatory for Human Rights, which relies on a broad network of sources across Syria, said the strike had hit army positions south of Albu Kamal, a town on the border with Iraq.

"At least three vehicles were destroyed by the strike," said Observatory chief Rami Abdel Rahman.

He said the 12 pro-regime fighters killed were not Syrians, but he could not provide more details on their identity.

A military source quoted by state news agency SANA said "some of our military positions between Albu Kamal and Hmeimeh were hit this morning in an aggression by American coalition warplanes".

The Pentagon denied the strike was carried out by the coalition fighting the Islamic State group.

"These reports are false, the coalition did not strike any Syrian army positions in eastern Syria," Pentagon spokesman Lieutenant Colonel Kone Faulkner said in a statement.

"The coalition's mission is to defeat ISIS (IS) in designated parts of Iraq and Syria, and to set conditions for follow-on stability operations. This mission has not changed."

Albu Kamal and Hmeimeh lie in Syria's eastern Deir Ezzor province, where Russian-backed Syrian troops and the US-led coalition have been waging separate offensives against IS.

A "de-confliction" line in place along the Euphrates River since last year is meant to keep the two assaults from crashing into each other.

Loyalist troops are present west of the river while the US-backed Syrian Democratic Forces (SDF) are on the east.

A military source from forces allied to Syria's government said the strikes had targeted two regime military positions near a frontline with IS.

- Repeated incidents -

"There are no Iranian or Lebanese fighters among the dead," the source said.

Iraqi Shiite militia are also fighting alongside Syrian regime forces in the area.

The Observatory said the government was sending reinforcements.

It was not immediately clear whether jihadists were active in the area at the time, nor whether the pro-regime casualties may have been accidental.

The coalition has carried out several deadly strikes against Syrian government forces and allied fighters in recent months.

In February, coalition bombing raids in Deir Ezzor province killed at least 100 regime and allied fighters, including Russian nationals, in retaliation for an attack on SDF positions.

And in September 2016, US-led strikes on regime military positions in the east left more than 60 Syrian troops dead. The coalition later said it had mistaken the fighters for IS jihadists.

The Islamic State group has lost nearly all the territory it once controlled in Syria and Iraq but it still holds some villages in the Euphrates Valley area.

On Monday, Syrian troops and allied forces ousted IS from the last districts it held in the capital Damascus.

After a fierce month-long battle, an evacuation deal saw the remaining jihadists bussed out of the city towards small pockets of land still held by IS in the Badiya, a vast desert area stretching from central Syria to its eastern border with Iraq.

The day after the transfer, IS fighters in the Badiya attacked a nearby government position, leaving more than two dozen Syrian troops and allied fighters dead.

The Observatory said the IS fighters responsible were from the group that had just been transferred out of the Yarmuk area in southern Damascus.

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