"CENTCOM continues to assess the outcome of the strike and has been made aware of allegations that the strike may have resulted in a civilian casualty," spokesman Major John Moore said.

The command is "investigating to determine whether or not the action may have unintentionally resulted in harm to civilians," Moore added.

On May 3, CENTCOM announced that it had targeted a "senior Al-Qaeda leader" in northwest Syria.

But hours later, the brother of Lotfi Hassan Masto -- a man killed in a strike on the outskirts of the village of Qurqania -- said he had no Al-Qaeda ties.

He was "not in Al-Qaeda and everything being said about him being in Al-Qaeda or in the Nusra Front is all lies upon lies. Even the American propaganda is all false," said Mohammed Masto, 72, referring to a group previously affiliated with Al-Qaeda.

Masto said his brother was around 60 years old, had 12 children, owned a chicken farm and "was tending to his sheep in the mountain when the aircraft came and targeted him."

"He was happy with his life and everyone loved him and appreciated him," he said. "He minded his own business and lived at the edge of the village."

The conflict in Syria has killed around half a million people since it began in 2011 with a brutal crackdown on anti-government protests, spiraling into a complex battlefield involving foreign armies, militias and jihadists.

The United States has around 1,000 troops deployed in Syria as part of international efforts to combat jihadists, and periodically carries out strikes targeting militants in the country.