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 52 pro-regime forces were killed in one of the deadliest air attacks in recent months.

"Among them are at least 30 Iraqi fighters and 16 Syrians, including soldiers and members of loyalist militias," Observatory chief Rami Abdel Rahman told AFP.

The nationalities of the remaining six fighters were not immediately known, he said. There are Iraqi, Iranian, Lebanese and even Afghan fighters stationed in the area.

According to Abdel Rahman, some wounded fighters were treated in the nearby town of Albu Kamal while others travelled across the border to Iraq.

A military source in Deir Ezzor told AFP the warplanes hit "joint Iraqi-Syrian positions in Al-Hari".

The attack was first reported by Syrian state media overnight, which cited a military source and accused 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.

- 'No strikes' -

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.

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

Iraqi warplanes also have occasionally bombed IS positions in Syria's 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.

- 'Notorious transit town' -

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," Observatory head 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.