Five Iran-backed fighters were killed on Sunday in a drone strike in Syria's east, near the Iraqi border, a war monitor said, adding it was not clear yet who was behind the attack.

"Five pro-Iranian fighters were killed and others were injured, some severely... after an unknown drone targeted the military vehicle they were in... near the Syrian-Iraqi border," said the Syrian Observatory for Human Rights.

The strike occurred in Syria's eastern Deir Ezzor province, where Iran wields significant influence and which is regularly targeted by Israel and the United States, according to the Britain-based monitor with a network of sources inside Syria.

In June, three pro-Iran fighters, including at least two Iraqis, were killed in an overnight air strike in eastern Syria near the Iraqi border, the Observatory had reported at the time.

Since the beginning of the Syrian civil war in 2011, Israel has carried out hundreds of strikes mainly targeting army positions and Iran-backed fighters including from Lebanon's Hezbollah group.

Israel rarely comments on individual strikes in Syria.

Iran-backed groups including Lebanon's powerful Hezbollah movement have bolstered President Bashar al-Assad's forces during Syria's civil war.

The Syrian government's brutal suppression of a 2011 uprising triggered the conflict that has killed more than half a million people and drawn in foreign armies and jihadists.