An air strike has killed nearly 40 pro-regime foreign fighters in eastern Syria, with a US-led coalition denying accusations from Damascus that it was behind the attack.

The strike just before midnight hit Al-Hari, a town controlled by regional militias fighting in the complex seven-year war on behalf of President Bashar al-Assad.

The Syrian Observatory for Human Rights, a Britain-based monitor of the conflict, said it was one of the deadliest air attacks on government loyalists in recent months.

"Thirty-eight non-Syrian fighters from regime loyalist militias were killed in the night-time raid on Al-Hari, on the Syrian-Iraqi border," said Observatory head Rami Abdel Rahman.

He could not give any further details on their nationalities, but there are Iraqi, Iranian, Lebanese and even Afghan fighters stationed in the area.

Syrian state media reported the attack overnight, citing a military source and accusing the US-led coalition fighting the Islamic State group of carrying it out.

It said several people were killed and wounded but did not give a specific number or their nationalities.

The coalition's press office said it had heard reports that a strike in the area had killed and wounded members of a pro-regime Iraqi militia, but denied it was responsible.

"There have been no strikes by US or coalition forces in that area," it told AFP by email.

IS overran large swathes of Syria and neighbouring Iraq in 2014, declaring an Islamic "caliphate" in areas under its control.

Separate offensives have since whittled down the jihadists' territory in Syria to just a handful of pockets in the eastern desert, including in the Deir Ezzor province where Al-Hari lies.

A US-backed alliance of Kurdish and Arab fighters and Russia-supported regime forces are carrying out separate operations against those IS-held pockets, and even Iraqi warplanes have occasionally bombed IS positions in Syria's east.

De-confliction line

The two forces have mostly avoided crashing into each other thanks to a de-confliction line that runs across the province along the winding Euphrates River.

Syrian troops are batting IS on the western river bank, while the US-backed Syrian Democratic Forces fight on the east.

Al-Hari lies on the western side, close to the river and the de-confliction line.

The buffer has largely been successful in keeping the two offensives apart, but there have been exceptions.

Last month a dozen pro-regime fighters were killed in an air strike on Syrian government positions that the Observatory and state media blamed on the coalition.

The Pentagon denied responsibility.

In February US-led coalition air strikes killed at least 100 pro-regime fighters in Deir Ezzor province, including Russians.

"The strike on Al-Hari produced the highest death toll for regime forces since the February incident," the Observatory's chief Abdel Rahman told AFP.

More than 350,000 people have been killed since Syria's conflict erupted in 2011 with protests against Assad's rule.

Those demonstrations spiralled into a full-blown war that has drawn in world powers and seen the rise of jihadist forces like IS.

The strike on Al-Hari came a day after the US-backed SDF announced it had ousted IS from Dashisha, a village to the north in Syria's Hasakeh province.

The village had been one of the last IS-controlled areas on a corridor linking Syria with Iraq.

"For the first time in four years, Dashisha, a notorious transit town for weapons, fighters, and suicide bombers between Iraq and Syria, is no longer controlled by ISIS terrorists," Brett McGurk, the US president's special envoy for the war against IS, said on Monday.