Dozens of Shia militiamen have been killed in an attack in Iraq close to the border with Syria, reports say.
There are conflicting accounts as to what happened. The group said US aircraft were to blame, though the US-led coalition denied this.
So-called Islamic State (IS) has said it killed scores of Iraqi troops in an ambush in the area.
Iranian-backed militia and the US-led coalition are fighting separate campaigns against IS in Iraq.
More than 30 fighters were killed and 85 wounded in the incident on Monday, reports say.
The Sayyed al-Shuhada militia, part of the government-allied Popular Mobilisation units (PMU), accused the US of deliberately attacking it. According to the militia, the US forces said in a statement that so-called smart bombs had been used to target IS positions in the area.
"We announce that this action will not pass unpunished," the militia said. "We call for the protection of the Iraqi-Syrian border and prevention of the Americans from exploiting these borders to pass their agenda."
However, a spokesman for the US-led coalition denied its forces had been involved.
On Monday, IS said its fighters had attacked Iraqi army positions near al-Tanf crossing on the Syria side of the border. The group said it killed 68 "apostates" and captured one soldier.
Despite losing Mosul in June, IS still controls territory in three areas of Iraq - around Hawija, 130km (80 miles) south-east of Mosul; around Tal Afar, 65km west, and from Ana to Al-Qaim in the Euphrates river valley, 250km to the south-west.