But a US official said there was cause to believe Israel carried out the deadly raid along the border with Iraq that hit forces battling on the side of Syrian President Bashar al-Assad.

Both Damascus and the Iraqi militia at first pointed the finger at the US-led coalition fighting the Islamic State group in the area.

Iraq's Hezbollah Brigades militia has said 22 of it fighters were killed in an air strike Sunday on a military base in eastern Syria that reportedly killed more than 50 people.

A powerful Iraqi militia backed by Iran pledged Thursday to get revenge for a deadly air raid on its fighters in Syria, blaming the United States or Israel for the attack.

Hezbollah Brigades spokesman Jaafar al-Husseini said it was still too early to say definitively whose forces carried out the strike, but insisted it "could only have been" the Americans or Israelis.

"When it becomes known who was responsible then there will be an appropriate response and the hand of the resistance will strike anywhere," he said during a memorial ceremony for the dead at a Baghdad mosque.

Israel has pledged repeatedly to do w

hatever it takes to stop Tehran or its "proxies" from building up their military presence in Syria and carried out large-scale strikes last month in the war-torn country.

The Shiite Hezbollah Brigades are part of the Hashed al-Shaabi paramilitary units that battled IS in Iraq under the command of the country's prime minister.

The group has also been fighting for Assad across the border in Syria independent of the authorities in Baghdad.

Iraq declared victory over IS late last year but the jihadists still control pockets of territory in Syria, where they are facing both US-supported forces and pro-regime troops.

A "de-confliction line" exists in eastern Syria to keep the rival forces confronting IS from coming into conflict with each other.

Sunday's strike hit a site that sits on a key route linking the Syrian-Iraqi border, and Iran beyond that, all the way west to the frontier with Lebanon.