Dozens of pro-Damascus fighters, including 22 from Iraq's PMU, were killed in attack near Albu Kamal in Deir Ezzor province
A US official said on Monday Washington suspects that Israel is behind the air raid that killed dozens of pro-Syrian government fighters in Deir Ezzor province.
"We have reasons to believe that it was an Israeli strike," the official told AFP on condition of anonymity following the Sunday night raid on Al-Hari, a town controlled by regional militias fighting alongside the Syrian government.
Damascus had accused Washington of being behind the attack; the US military denied any involvement. Iraq's paramilitary group the Popular Mobilisation Units, which said it lost 22 fighters in the raid, backed the Syrian government's claim that US aircraft carried out the strike.
"At 22:00 last night a US plane hit a fixed headquarters of the Popular Mobilisation Units' 45th and 46th brigades defending the border strip with Syria, using two guided missiles which led to the martyrdom of 22 fighters," the Iran-backed paramilitary group said in a statement.
It demanded an explanation from the United States.
An Iraqi military statement later said no Popular Mobilisation Forces or other Iraqi troops tasked with securing the Iraqi-Syrian border had been hit by the air strike, and it had taken place inside Syria.
"No member of the US-led coalition carried out strikes near Albu Kamal," Major Josh Jacques, a US Central Command spokesman, told Reuters.
The Britain-based Syrian Observatory for Human Rights said the strike was one of the deadliest on forces allied with Syria's government. According to the Observatory, 52 fighters were killed in the attack.
The attack took place in al-Hari, southeast of the town of Albu Kamal, state news agency SANA said, citing a military source.
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Israel has repeated targeted pro-government forces in Syria throughout the past three years. The Israeli military rarely acknowledges its activities in Syria.
US-backed fighters and Russia-supported government forces are carrying out separate operations against small pockets of territory held by the Islamic State (IS) group in Deir Ezzor province, where Al-Hari lies.
Both sides have mostly avoided running into each other, and a de-confliction line exists to prevent such incidents, but there have been exceptions.
In May, a dozen pro-government fighters were killed in an air strike on Syrian army positions that the monitor and Syrian state media said was carried out by the US-led coalition.
The Pentagon denied responsibility then.
Deadly clashes also broke out in April, but the bloodiest incident yet was in February, when the US-led coalition carried out air strikes that killed at least 100 pro-government fighters in eastern Syria, a number of whom were reported to be Russian mercenaries.