An air strike has killed more than 50 pro-regime fighters in eastern Syria, most of them foreign, with the US-led coalition denying accusations from Damascus 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 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 said.
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 Mr 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 said 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 ISIS 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 said.
ISIS 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 extremists' 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 ISIS-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 ISIS on the western river bank, while the US-backed Syrian Democratic Forces fight on the east.
Iraqi warplanes also have occasionally bombed ISIS 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.
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 said.
More than 350,000 people have been killed since Syria's conflict erupted in 2011 with protests against Mr Assad's rule.
Those demonstrations spiralled into a full-blown war that has drawn in world powers and seen the rise of extremist forces like ISIS.
The strike on Al Hari came a day after the US-backed SDF announced it had ousted ISIS from Dashisha, a village to the north in Syria's Hasakeh province.
The village had been one of the last ISIS-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 ISIS, said on Monday.