A strike on Monday near Iraq's western border with Syria killed at least four fighters from a former coalition, two security officials said.
The fighters from the Popular Mobilization 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, AFP reported.
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.
Since the start of the Middle East war on February 28, bases belonging to PMF 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.