By Yeganeh Torbati and David Alexander

WASHINGTON (Reuters) -- A U.S. air strike that targeted an Islamic State checkpoint in Iraq in March likely killed four civilians, one of whom may have been a child, the U.S. military said on Friday in a rare statement acknowledging the death of civilians.

An investigation of the March 13 strike found the checkpoint was a legitimate target, that "all reasonable measures" were taken to avoid unintended deaths and the presence of civilians could not be detected until after the warplane had launched its weapons.

Air Force officials at U.S. Central Command began investigating the air strike by an A-10 tank-killer jet near al Hatra, Iraq, after an Iraqi woman sent an email claiming her car and a second vehicle had been destroyed and the passengers killed. She sought compensation for her vehicle.

"The preponderance of the evidence gathered during the investigation indicates that the air strikes likely resulted in the deaths of four non-combatants," the military said in a statement.

The person who notified the U.S. military about the incident said two women and three children had been killed. A U.S. military analysis indicated one of the people was much smaller than the rest and may have been a child.

But the military said in a statement it could not be certain about the age or gender of those killed without forensic examination or other evidence not available to the coalition.

The U.S. military has rarely acknowledged causing civilian casualties in the fight against Islamic State militants, with Friday's announcement bringing the total to six. In May, it concluded two children had been killed in an air strike in Syria in November 2014. It is investigating a few dozen other reports.

"If we knew there were civilians we would not have conducted a strike," Air Force Colonel Patrick Ryder, a spokesman for U.S. Central Command, said at a briefing on Friday.

The killings in March occurred when a U.S. warplane attacked an Islamic State checkpoint that had been "thoroughly reviewed and validated" as a target, according to an investigative report released by the U.S. military.

As the warplane prepared to attack, a GMC sport-utility vehicle and a black Kia sedan arrived at the checkpoint, the report said. The drivers got out of their vehicles and spoke to militants manning the checkpoint for about 40 minutes, it said.

Several other vehicles pulled up to the checkpoint during that time and continued their journey after a brief stop. The aircrew executing the strike relayed that information to the Combined Air Operations Center that coordinates military air traffic in the region.

"Based upon the actions on the ground by the personnel at the checkpoint, the aircrew and CAOC personnel assessed that the drivers and vehicles, as well as the checkpoint, were ISIL and therefore lawful targets," the report said, using an acronym for Islamic State.

An analysis of video footage taken by the warplane's weapons systems showed that four people, whose presence was previously unknown, got out of the two vehicles after "the aircrews had released weapons on the target and immediately before the weapons impacted the target area," the report said.

"Video footage review indicates the aircrew had no opportunity to detect the presence of the likely civilians in the target area prior to weapons impact," the investigation found.

(Reporting by Yeganeh Torbati; Writing by David Alexander; Editing by Doina Chiacu and Bill Trott)