The helicopter crashed southwest of the south central district of Daychopan in Zabul province on Sunday, the US military said in a statement, adding that the cause of the crash was unknown.
"US military ground forces are at the crash scene providing security, and recovery operations are ongoing," it said.
Colonel Jim Yonts said the crash came after the helicopter had dropped off troops as part of an ongoing operation against suspected fighters.
He said it was too early to say if hostile fire or mechanical failure was to blame.
"There is no indication at this time that this is a result of hostile fire," US military spokesman Lieutenant Colonel Jerry O'Hara told the Associated Press.
"There are no survivors."
Gulab Shah, a spokesman for Zabul's governor, said there had been no fighting in the area at the time. He said he had been told by US forces in the region that the chopper crashed while returning to a US base after dropping off troops for a raid on a suspected militant target.
Daychopan is about 300km southwest of the capital, Kabul, and has been the scene of some of the fiercest fighting with Taliban rebels.