WASHINGTON -- A U.S. Marine Corps helicopter was declared missing on Tuesday with six Marines and two Nepalese soldiers aboard as it was ferrying rice and tarps to earthquake victims in rugged terrain in Nepal, near a town hard hit by the latest deadly aftershocks.

Army Colonel Steve Warren, a Pentagon spokesman, said reports from the field indicated the helicopter's crew was overheard talking about fuel problems before U.S. military officials lost contact with the light attack helicopter.

A 90-minute search by three Marine Corps V-22 tilt-rotor aircraft failed to locate the aircraft before nightfall and the air search was suspended. A ground search by Nepalese troops was continuing, Warren said.

"Essentially what we have right now is truly a missing helicopter. We simply don't know its location," Warren said, noting that the area is "rugged and mountainous" and any radio transmissions from the helicopter might not be heard due to the mountains.

The helicopter was one of three Marine Corps UH-1Y Hueys participating in earthquake relief operations following last month's 7.8 magnitude tremor. It has been followed by aftershocks, including a major 7.3 magnitude tremor earlier on Tuesday.

Army Major Dave Eastburn, a spokesman for U.S. Pacific Command, said the helicopter was carrying the U.S. and Nepalese forces when it went missing in the area of Charikot village, which was in the area hardest hit by Tuesday's aftershock.

Warren said an Indian helicopter working in the quake zone had overheard radio chatter from the U.S. helicopter about "fuel issues" before it went missing. Warren said it was not clear whether the helicopter might have been low on fuel or if there was a problem with its fuel lines.

"The UH-1 had launched to deliver tarps and rice," Warren said. "Because of the terrain (it) had not been in contact for approximately two hours. No emergency beacon has been detected at this time."

Warren said Marine Brigadier General Paul Kennedy, deputy commander of the III Marine Expeditionary Force, was directing the rescue effort. U.S. Air Force pararescue forces, who could be air-dropped into the area, had rehearsed and were available if needed, Warren said.

A Nepalese air brigade reported seeing the U.S. helicopter at a location some 40 miles (65 km) east of the capital Kathmandu, but the three aircraft sent to search the area did not spot it during the hunt before dark, Warren said.