Four civilians, including possibly a child, were “likely” killed in a US airstrike against an Islamic State group checkpoint in Iraq in March, according to a US military investigation.

It marks only the second such concession since the start of a coalition air campaign in Iraq and Syria – the US military in November 2014 admitted accidentally killing two children during a strike in Syria.

The US military, which for more than a year has led the coalition bombing Isis extremists’ positions in the two countries, investigated the Iraq incident after a woman said her car had been destroyed with five civilians in it.

“The preponderance of the evidence gathered during the investigation indicates that the air(strike) likely resulted in the deaths of four non-combatants,” the US military’s Central Command said in a statement on Friday.

“One of the non-combatants may have been a child.”

Officials say a US A-10 air-to-ground combat plane targeted the checkpoint on 13 March, near an Isis-held area of Hatra in northern Iraq.

“However, before coalition air forces could complete the air strike, two vehicles arrived at the checkpoint and parked within the target area,” the investigation states.

Because the drivers of the cars were talking to people at the checkpoint for about 40 minutes while other vehicles drove through, the US pilot and supervisors decided the drivers were “Isil and therefore lawful targets”.

The A-10 air crew did not realize four “additional personnel” had been in the vehicles.

According to the report, it was not possible to definitively prove the gender or age of the victim who might have been a child.

Colonel Pat Ryder said that 26 other similar cases or allegations are under various stages of review.

“We regret the unintentional loss of lives and keep those families in our thoughts,” Lieutenant General CQ Brown of US Air Forces Central Command said.

“Our goal is to defeat Daesh (the Isis group), a terrorist organization that continuously wraps itself around the population, and we do everything we can to prevent unintended deaths or injuries to non-combatants.”