The fighters from the group -- also known as the Popular Mobilisation Forces (PMF), now part of Iraq's regular army -- "were killed and three others were wounded" in the late afternoon attack on a checkpoint at the entrance to the city of al-Qaim, a local security official said.

An official with the PMF, which includes pro-Iranian groups, put the toll higher, at six dead, blaming the United States for the strike.

He said the checkpoint, which also housed army and police personnel, was targeted again when ambulances arrived to help victims.

Iraq has recently regained a sense of stability following years of conflict, and was unwillingly drawn into the current Middle East war after having long been a proxy battleground between the US and Iran.

The Hashed al-Shaabi is an alliance of factions created in 2014 to fight jihadists and is now integrated into the Iraqi armed forces.

Iran-backed groups have brigades that operate within the Hashed al-Shaabi, but have a reputation for acting on their own.

Since the start of the Middle East war on February 28, bases belonging to Hashed al-Shaabi have been hit several times, with strikes mostly targeting Tehran-backed armed groups.

These groups are also united under a loose alliance called the Islamic Resistance in Iraq, which has claimed attacks against US bases in Iraq.