The US-led coalition is investigating reports that an air strike mistakenly targeted pro-government Iraqi forces who were taking part in the fight against Isis near Mosul.
At least 20 Sunni tribesmen were reportedly killed in the incident, which took place near the town of Qayyara 60 km from Mosul, where preparations are underway to retake the city from the Islamists.
According to local commander Sheikh Nazhan Sakhr al-Lihaybi, the soldiers had just taken part in an operation to repel an advance by Isis when they were targeted by jets. It is still unclear if the planes were Iraqi or from the US-led coalition.
A senior US defence official told AFP: "This most likely was from a coalition air strike. Right now, we are still getting information."
He added that the incident, which took place at around 1am local time on Wednesday (5 October) is presumed to have been "what we unfortunately call a blue-on-green incident."
Canadian Brigadier General D.J. Anderson said the coalition was investigating the incident.
"We are aware of the alleged reports that coalition forces mistakenly fired on Sunni tribal fighters," Anderson said. "We are looking into this to determine the facts that surround the case" Metro US reported.
In Syria, there have been several similar incidents, including the bombing of Syrian government forces by the US on 17 September in which at least 62 soldiers were killed. In Iraq a US bombing raid killed dozens of government troops in Anbar Province in December 2015.
This latest incident comes as the Iraqi government, Kurds, local militias and the coalition prepare to try to wrest Mosul from the grasp of Isis, which overran the city of 1.5 million in 2014.
Today Sky News reported Kurds engaged in the battle have vowed to hold on to land they seize during the fighting, a claim which has reportedly caused consternation in Baghdad.