BAGHDAD (Reuters) - Nine fighters from a Shi’ite Muslim militia battling Islamic State were killed in northern Iraq when an Iraqi army aircraft fired at them in error, security and militia sources said on Sunday.

The fighters were responding to an Islamic State attack west of Camp Speicher, a former U.S. base outside the city of Tikrit, said Ahmed al-Assadi, spokesman for the Hashid Shaabi, a coalition of mostly Shi’ite militias fighting the militants alongside Iraq’s military.

More than a dozen fighters were wounded, he said.

Colonel Mohammed al-Assadi, spokesman for the joint police and military operations command in Salahuddin province, said that at 10:30 pm (1930 GMT) on Saturday, an Iraqi army aviation drone opened fire due to mistaken coordinates.

Assadi said the drone was being fired at from the ground and “fired on the advancing Jund al-Imam forces, killing nine and wounding around 15,” he added, referring to the militia.

Spokesmen for Iraq’s defense ministry and joint operations command were not immediately available for comment.

Asaib Ahl al-Haq, a powerful Iranian-backed militia that is part of the Hashid, condemned the incident and blamed it on the U.S.-led coalition bombing Islamic State targets in Iraq and neighboring Syria.

“The American coalition renewed its attacks on the Hashid Shaabi resistance factions when an American drone bombed the headquarters of Kataib Jund al-Islam at Camp Speicher,” spokesman Naim al-Uboudi said in a statement.

But the coalition said in a daily statement it had not bombed any targets in the area, and its Baghdad-based spokesman, U.S. Army Colonel Steve Warren, denied coalition aircraft were responsible.

“It was Iraqi for sure,” he said by phone.

The fight against Islamic State is testing a thorny arrangement that puts the U.S.-led air campaign on the same side as Iranian-backed militias supporting Iraqi forces on the ground.

Washington listed some of the armed groups as “terrorist” organizations after they battled U.S. forces following the 2003 invasion. The militias, in turn, perceive in continued American involvement in Iraq a plan to partition the country.

Nine Iraqi soldiers were killed last month when coalition war planes, relying on information from Iraqi security forces, mistakenly struck at troops near the city of Falluja, 50 kilometers (30 miles) west of Baghdad.