MOSCOW Russia's most powerful mercenary on Thursday said he was sending back the bodies of a U.S. citizen who was killed in fighting in the Ukrainian city of Bakhmut and that of a Turkish citizen who was found dead in a blown up building.

Mercenaries from Russia's Wagner Group were shown in a video clip nailing closed the two wooden coffins and then draping a U.S. "Stars and Stripes" flag over one coffin and the "red banner" flag of Turkey on the other.

"We are returning the body of an American and the body of a citizen of Turkey along with prisoners," Wagner founder Yevgeny Prigozhin said, standing near the coffins.

The U.S. citizen perished in fighting in a part of Bakhmut known as the "nest" while the Turk was found dead along with a Turkish woman in the remains of a building blown up Ukrainian forces, Prigozhin said.

The Turkish woman's body could not be recovered, Prigozhin said.

Wagner named the American as Nicholas Maimer, a former U.S. special forces soldier.

The death toll in the battle for Bakhmut, known as the meat grinder, has been monstrous. Prigozhin said more soldiers were killed in the battle for the city than in the Soviet Union's entire 10-year war in Afghanistan (1979-1989).

Prigozhin said that in Bakhmut, Ukraine had suffered casualties of 50,000-70,000 wounded and 50,000 dead. He said around 20,000 of his own mercenaries had been killed during the war.

The United States has estimated Russia has suffered 100,000 casualties in Bakhmut including more than 20,000 dead.

Neither Russia nor Ukraine release figures on their own casualties. Ukraine has said Russian losses are far higher than its losses.

(Reporting by Guy Faulconbridge; Editing by Andrew Osborn)