An airstrike that killed more than 50 pro-regime fighters in eastern Syria, most of them foreign, was carried out by Israel, not the United States, a US official told CNN Monday.

A US source also told AFP: “We have reasons to believe that it was an Israeli strike.”

Damascus has accused the US-led coalition of carrying out the late night June 17 strike on Al-Hari, a town near the Iraqi border controlled by regional militias fighting in the complex seven-year war on behalf of Syrian President Bashar Assad.

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An Iraqi paramilitary force said 22 of its fighters were killed in the raid, and also accused the United States of carrying out the strike.

“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 Mobilization Units).

The US has denied any coalition aircraft were in the area at the time of the strike.

Israel declined to comment, but a strike so far from its border would veer from most other strikes in Syria attributed to Israel, which have largely taken place closer to Syria’s borders with Israel and Lebanon.

The target, apparently Shiite Iraqi militia fighters loyal to Assad, would also mark a shift for Israel, which has previously only carried out airstrikes against Iran’s forces and its proxies, according to reports.

At a cabinet meeting on Sunday Israel’s Prime Minister Benjamin Netanyahu warned that Israel was “taking action — against efforts to establish a military presence by Iran and its proxies in Syria both close to the border and deep inside Syria. We will act against these efforts anywhere in Syria.”

The Syrian Observatory for Human Rights, a Britain-based monitor of the conflict, said 52 pro-regime forces were killed in the air attack — one of the deadliest 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 traveled 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 terror 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.

AFP contributed to this report.