"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," said the Iran-backed Hashed (Popular Mobilisation Units).

It said the raid took place "700 metres (yards) inside Syria", adding that an investigation had been opened and the results would be passed on to Prime Minister Haider al-Abadi.

The Syrian Observatory for Human Rights said earlier that more than 50 fighters allied to the Damascus regime, most of them foreign, were killed in Sunday night's raid on Al-Hari in eastern Syria.

It did not say who carried out the attack.

Syrian state media, citing a military source, accused the US-led coalition fighting the Islamic State jihadist group and said the attack left several dead and wounded, without giving precise figures.

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

The Hashed said its fighters were deployed inside Syria north of the Albu Kamal border town "because of the desert nature of the zone and for military imperatives to prevent terrorist infiltration into Iraq".

The bodies of at least three fighters of the Hezbollah Brigades, part of the Hashed coalition fighting IS, have been repatriated to the southern Iraqi province of Zi Qar, an AFP correspondent said.