A US Marine Corps helicopter carrying out earthquake relief in Nepal was reported missing on Tuesday with eight personnel on board, officials said.
"The incident occurred near Charikot, Nepal while the aircraft was conducting humanitarian assistance and disaster relief operations," said Major Dave Eastburn, spokesman for US Pacific Command.
The UH-1Y Huey, which was carrying tarps and rice, had just dropped off supplies and was headed to another destination, officials said.
Before the chopper went missing, there was "some chatter about a fuel problem" on the radio from the helicopter crew, Pentagon spokesman Colonel Steven Warren said.
The chopper was carrying six US Marines and two soldiers from the Nepalese army, officials said.
The Huey helicopter was last seen west of Charikot but a search of the area was unsuccessful.
A search by aircraft before nightfall had not spotted any sign of smoke or any crash site, raising hopes the helicopter may have made a safe emergency landing, he said.
The Nepalese army was conducting a search on the ground but aircraft could not fly until daylight to resume the search from the air.
Warren said that "no emergency beacon has been detected at this time" from the helicopter crew.
Due to the rugged, mountainous terrain, even if a beacon signal was switched on it might not be picked up easily as it requires a clear line of sight, he added.
About 300 US military troops have deployed to support the relief effort in Nepal, where aftershocks continue to claim lives after last month's massive quake.
A new earthquake and powerful aftershocks killed dozens of people and brought fresh terror to Nepal on Tuesday.
The helicopter was among more than a dozen US military aircraft devoted to aid operations, including two other Huey choppers and four tilt-rotor Ospreys as well as cargo planes.
Relief teams from around the world are still working to provide water, food and medical assistance to Nepalis after the April 25 quake -- the country's deadliest in more than 80 years.