WASHINGTON—A U.S. military helicopter conducting relief efforts in Nepal has been declared missing, and was the subject of a U.S. search, military officials said Tuesday.

Army Maj. Dave Eastburn, a spokesman for U.S. Pacific Command, said a UH-1Y Huey helicopter was declared missing at 9 a.m. EDT.

There were six U.S. Marines and two Nepalese soldiers aboard the helicopter when it disappeared, military officials said.

The U.S. began an aerial search for the missing helicopter, using tilt-rotor V-22 Osprey aircraft and other helicopters, while the Nepalese began a ground search. As night fell in Nepal, the U.S. suspended the air search, but officials said it would resume at daybreak. As of Wednesday morning in Nepal, the helicopter still hadn’t been found.

Military officials couldn’t immediately confirm whether the helicopter was believed to have crashed. But as hours passed without contact from the helicopter or crew, officials grew more pessimistic, suspecting the aircraft had crashed.