Somalia's government had asked the United States to explain the strike, which it said had been conducted against forces of the semi-autonomous, northern region of Galmudug.

The errant strike illustrated the perils of Washington's efforts to battle al Shabaab, an al Qaeda-aligned group, by working with armed Somali factions that are often feuding.

Shabaab has been responsible for numerous attacks, including the September 2013 siege of Kenya's Westgate shopping mall that left at least 67 dead.

The day after the Sept. 28 U.S. strike in Somalia, officials in Galmudug accused a rival region, Puntland, of duping the United States into believing members of its security forces were in fact Islamist rebels.

An al Shabaab spokesman told Reuters at the time it did not have any fighters in the area of the strike.

The draft statement by the U.S. military's Africa Command said the air strike was carried out at the request of Puntland Security Forces "and our own assessment of the situation."

A PSF-led patrol had come under attack by a group of armed fighters and in response, "the U.S. conducted a self-defense strike to neutralize the threat, killing 10 armed fighters and wounding three others," the statement said.

A review of the strike, which began Oct. 4, determined that "The armed fighters were initially believed to be al-Shabaab but with further review it was determined they were local militia forces," it said.

"Operating under legal authorities, U.S. forces lawfully utilized self-defense to support the PSF in response to hostile actions conducted by the armed group against a partnered force," the review concluded. "No U.S. forces were killed or injured as a result of this incident."

(Editing by Jonathan Oatis and David Gregorio)