BAGHDAD (AP) — A coalition air strike mistakenly hit friendly Iraqi forces and civilians in western Anbar Saturday, according to senior Iraqi officers, but the U.S.-led coalition said the matter is still under investigation.

The military aircraft mistakenly fired at a gathering of Iraqi tribal fighters allied with the central government and civilians in western Anbar, killing seven and wounding 11 others, according to two senior Iraqi officers.

The officers spoke on condition of anonymity because they were not authorized to speak to media.

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The strike occurred after U.S.-backed Iraqi forces arrested an Islamic State group operative in the early hours of Saturday morning, according to accounts from both the coalition and the Iraqi officers.

“During the search and evidence collection the force was attacked by hand grenades from nearby homes which they responded to with fire and then withdrew from the area,” read a coalition statement provided to The Associated Press by email.

“While returning to their base the force noticed an unknown armed formation,” the statement continued, “Coalition air assets responded at 0200 and engaged the unknown group.”

The mistaken strike occurred because “coordination was not at the required level,” said the ministry of defense spokesman, Brig. Gen. Yahya Rasool, without elaborating.

The Iraqi prime minister’s office also confirmed Saturday that an investigation is underway.

Iraqi Shiite firebrand cleric Muqtada al-Sadr condemned the attack as a “flagrant violation” against the Iraqi government and its independence and sovereignty. Al-Sadr called for “immediately punishing the aggressors,” in a statement posted to Twitter.

The U.S.-led coalition has closely supported Iraqi military advances against IS with air strikes for over three years. Last month Iraq declared victory over the extremists but continues to fight IS insurgent groups across the country.