Air strikes from unidentified drones killed three pro-Iran fighters in Syria's eastern province of Deir Ezzor near the Iraqi border, an anti-regime war monitor said on Wednesday.

The drones late Tuesday targeted trucks of the Iraqi paramilitary network Hashed al-Shaabi after they had crossed the border into the Syrian border district of Albu Kamal, the British-based Syrian Observatory for Human Rights said.

Three were killed and several severely wounded, the monitoring group said.

A source within the Hashed al-Shaabi in Iraq however said that "the strikes destroyed four vehicles, but no one was killed".

"The site targeted was near a border post of the factions on the Iraqi-Syrian border," the source told AFP.

The Fatah alliance, the political wing of the Hashed al-Shaabi, condemned an "abject attack" on its forces on the frontier.