Sunday night's attack on the town of al Hari in Deir Ezzor, near the border with Iraq, killed 52 fighters, including 22 members of an Iraqi Shia militia, the Syrian Observatory for Human Rights monitoring group said.
Syrian state news blamed the US-led coalition fighting Isis for the casualties and Damascus has demanded an explanation for the targeting from Washington. Baghdad, too, condemned the attack on Iraqi paramilitary forces, calling it “support for Isis”.
US Central Command denied responsibility, however, saying in a statement there were “no strikes by US or Coalition forces in that area during the time in question”.
Instead, unnamed US officials told AFP and CNN that the strike was carried out by the Israeli air force – a move that would represent several new developments for Israeli involvement in neighbouring Syria’s civil war.
The Israel Defence Forces (IDF) did not comment on the strike, as per its usual policy.
If the IDF was indeed behind the attack, it is the first time Israel has targeted Iraqi Shia forces in the country, and hundreds of miles away from the area it normally operates in, which already has a heavy US, Russian and Syrian air force presence.
“We will take action – and are already 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,” Israeli Prime Minister Benjamin Netanyahu said on Sunday at a cabinet meeting.
Though formally neutral on Syria’s complex civil war, Israel is believed to have carried out more than 100 strikes in the south and west of the country in the last few years to prevent weaponry shipments to Lebanese militant organisation Hezbollah, which, along with troops belonging to Shia militias from Iran and Iraq, fights alongside Syrian President Bashar al-Assad’s troops.
As the tide of the seven-year-old war has slowly turned in Assad’s favour, tensions between Iran and Israel have ratcheted up, with Israeli officials warning they will not accept a permanent Iranian military presence in Syria.
Last month, the hostility between the two regional powers came to a head when Israel launched its most intensive attack on Iranian positions to date after what the IDF said was the first ever direct Iranian rocket attack on its troops in the occupied Golan Heights.
The confrontation, in which at least 23 people were killed, marked the most significant military skirmish between the two enemies amid a backdrop of escalating regional tensions.
In Syria’s east, US-led coalition, Russian and Syrian planes carry out airstrikes against Isis targets.
While the US uses a de-confliction line to avoid run-ins with the other parties, in May it is believed a US strike killed at least 12 pro-government fighters.
The bloodiest incident occurred in February, when at least 100 pro-government forces, including Russian mercenaries, were killed in a US-led coalition airstrike in Deir Ezzor.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies