BAGHDAD (Reuters) - A group of gunmen killed in U.S. airstrikes in Iraq last week were pro-U.S. fighters, an American military officer said on Sunday, despite the military’s public statements that they were insurgents.

The officer, who spoke on condition of anonymity, said U.S. military officials had talked to Sunni Arab tribal sheikhs in Taji, just north of Baghdad, to express their regret for the loss of life in the attack, which took place last Tuesday.

“There was some confusion and we were not able to turn off the attack quickly enough,” he said of the airstrikes that continued for several hours despite frantic phone calls from local tribal leaders to the U.S. base in Taji.

“We have talked to them and explained our sorrow over the incident and the loss of lives of volunteers trying to bring order to their neighbourhoods,” the officer said.

The incident threatens to derail a carefully constructed relationship between U.S. forces and anti-al Qaeda Sunni tribes in Taji and has put the spotlight on operating procedures for tribal police units the U.S. military is forming around Iraq.

“If they (the U.S. military) do not give us a proper reason for what happened, we will withdraw from the Awakening Council and let al Qaeda return,” said Sheikh Shathir Abid Salim, leader of the anti-al Qaeda group. His brother was among those killed.

The military said in a statement last week that it killed 25 suspected insurgents in operations targeting al Qaeda militants near the capital. Tribal leaders told Reuters U.S. warplanes had mistakenly bombed their men, killing 45.

The U.S. military officer told Reuters the men targeted in the airstrikes were a mixture of volunteers from the Taji Awakening Council and tribesmen loyal to Sheikh Shathir.

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He said U.S. forces in Taji, elements of the 1st Brigade, 1st Cavalry Division had signed a contract with the Awakening Council for tribesmen to man checkpoints in their battlespace.

Sheikh Shathir had carried out “a personal mission outside the scope of the contract”, sending his tribesmen outside the 1st Brigade’s battlespace to set up roadblocks in an area controlled by another U.S. military unit, he said.

“They were well-intentioned, trying to do what was best,” the officer said.

What happened next is still unclear but it appears that the neighbouring U.S. military unit, unaware they were members of the Taji Awakening Council, then identified the tribesmen as enemy fighters and called in airstrikes to kill them.

U.S. military spokesman Rear Admiral Greg Smith told reporters in Baghdad on Sunday, however, that what happened was “no accidental attack against what we identified as terrorists”.

“Our understanding to date is that the individuals that were killed and those that were captured were all participating in an activity that we deemed to be not in the best interest of Iraq,” Smith said.

The spokesman for the 1st Brigade, 1st Cavalry Division, the U.S. military unit in the Taji area, had said earlier on Sunday that the incident was still under investigation.

“The sheikhs told us that it was members of their tribe. We are investigating to find out exactly what occurred,” Major Randall Baucomb said.

The Taji Awakening Council is modelled on a similar council formed by Sunni Arab tribes in Iraq’s western Anbar province last year to fight Sunni Islamist al Qaeda. Since then, many similar councils have been set up in other areas of Iraq.

In partnership with the U.S. military, the councils have formed 186 “concerned local citizens” groups (CLCs), recruiting more than 70,000 men who play a key role in the new U.S. counter-insurgency strategy to combat al Qaeda.

“If there are CLCs who were caught up in that particular incident on that particular day, that’s unfortunate,” Smith said on Sunday. “It remains to be seen exactly what role the Taji CLC group was playing on that day.”