More than 20 fighters from an Iraqi paramilitary force key to the battle against the Islamic State group were killed in an eastern Syria air raid the United States linked to Israel.
The bombing raid hit Al-Hari, a town controlled by regional militias fighting in Syria's complex seven-year war alongside President Bashar al-Assad's forces.
Both Syrian authorities and Iraqi forces pointed the finger at the US-led coalition, which denied it was involved in Sunday night's attack.
"We have reasons to believe that it was an Israeli strike," a US official told AFP on condition of anonymity on Monday.
The raid slammed into a regime-controlled position in the border town and left at least 52 fighters dead, according to a Britain-based monitor.
Among them were fighters from Iraq's powerful Hashed al-Shaabi military alliance, some of whom have crossed into Syria to fight against IS.
The Iran-backed Hashed claimed that "US planes fired two guided missiles at a fixed position of Hashed al-Shaabi units on the border with Syria, killing 22 fighters and wounding 12."
The bodies of three Iraqi fighters killed in the raid were returned to their hometowns for burial, said AFP's correspondent in the southern Iraqi city of Nasiriyah.
The Syrian Observatory for Human Rights said a total of 30 Iraqi forces were among the dead in Al-Hari, as well as 16 Syrian forces and six unidentified fighters.
- 'No strikes' -
The attack was first reported by Syrian state media, which cited a military source accusing the coalition of bombing one of its positions in Al-Hari.
It said several people were killed and wounded but did not give a specific number or their nationalities.
A military source in Syria's Deir Ezzor province where the targeted area lies later said coalition warplanes hit "joint Iraqi-Syrian positions in Al-Hari".
The coalition's press office said it had received reports of a strike in the area that had killed and wounded Iraqi fighters, but denied it was involved.
"There have been no strikes by US or coalition forces in that area," it said in an email.
Hashed said its fighters had been deployed along the porous frontier with Syria on the orders of the Iraqi authorities.
However, late Monday the Iraqi military command denied it had positioned forces in Syrian territory, implying the dead fighters had acted without its consent.
Regretting the deaths, the command said it had been assured by the coalition that it was not responsible for the strikes.
Hashed is vital to the fight against IS in Iraq, but has also battled the jihadists across the border in their eastern Syria bastions.
Al-Hari is in oil-rich Deir Ezzor province where a US-backed alliance of Kurdish and Arab fighters and Russia-supported Syrian regime forces are carrying out separate operations against IS.
The jihadists have lost most of the territory they controlled in Syria and Iraq but remain in pockets of the eastern desert area including Deir Ezzor.
The US and Russian-backed forces have mostly avoided each other thanks to a de-confliction line that runs across the province along the winding Euphrates River.
- 'Highest toll' -
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 eastern Syria.
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.
The deadliest incident was in February, when 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.
Syria's conflict began in 2011 with protests against Assad, but then spiralled into a full-blown war that has drawn in world powers and given rise to jihadists 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 in 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 (IS) terrorists," said Brett McGurk, the US president's special envoy for the war against IS.