The U.S. military says an airstrike on an Islamic State checkpoint in Iraq in March probably killed four civilians, and one of them may have been a child.

In a rare statement acknowledging civilian deaths, the U.S. Air Forces Central Command said it investigated the incident after a woman said her car had been destroyed with civilians inside.

U.S. military investigators concluded that the Iraqi checkpoint was a valid target and said all reasonable measures were taken to avoid civilian causalities.

They said as the strike was about to take place, two vehicles arrived at the checkpoint, near an Islamic State-held area of Hatra. Investigators say the drivers got out of the vehicles and talked to the checkpoint personnel for about 40 minutes while other vehicles drove through, indicating that the drivers were associated with IS militants and were therefore valid targets.

'Unintentional loss of lives'

A military spokesman, Col. Patrick Ryder, told reporters that four other people, likely civilians, later emerged from the two vehicles parked at the checkpoint. He said they only left their vehicles after an A-10 attack plane had already released its weapons. He said the pilots did not realize there had been others in the vehicles.

In a statement, Lieutenant General Charles Q. Brown of U.S. Air Forces Central Command said, "we regret the unintentional loss of lives and keep those families in our thoughts."

He said the U.S. goal is to defeat the Islamic State group, which "continuously wraps itself around the population, and we do everything we can to prevent unintended deaths or injuries to non-combatants."

According to the report, the U.S. military said it could not definitively prove the age of those killed in the attack.

The military is investigating several dozen other strikes in which civilians were reportedly killed in either Iraq or Syria.