US Helicopter Missing After Nepal Earthquake: Still No Trace

"Where they went down, whether it got damaged," may explain no signal: official.

The head of the U.S. military’s joint task force in Nepal said the biggest challenge is the mountainous terrain where the UH-1 Huey may have touched down.

“It’s just a matter of receiving the signal,” said Maj. Gen. John Wissler. "Where they went down, whether it got damaged during the set-down process ... there are a number of reasons why those beacons and so forth would not operate.”

U.S. Marine and Air Force search teams were joined by ground troops from the Nepalese military. The search ended Wednesday night because of darkness and will resume at first light on Thursday.

The helicopter was carrying six U.S. Marines and two Nepalese service members, and had been delivering humanitarian aid in Charikot, an area 50 miles east of Nepal's capital of Kathmandu, close to the epicenter of Tuesday's large earthquake.

Wissler insisted the search for the missing helicopter is not detracting from the ongoing mission to help people affected by Tuesday’s earthquake.

He said he is not concerned that the U.S. military helicopter may be close to the Chinese border.

“Our sector is to the east of Kathmandu and extends up north, where the rescue and relief efforts are being coordinated by the Chinese,” Wissler said.

At Kathmandu’s international airport, U.S. helicopters like the one that went missing continued shuttling patients from hard-hit rural areas.

Forty-four people were airlifted Tuesday, and 15 others have been flown from the mountainous regions to the airport for immediate medical care, Air Force Col. Lee Anderson said.

Anderson added that many of those patients suffered from a number of injuries.

“Mostly crush injures, broken bones, banged up, head injuries, lacerations,” Anderson said.

ABC News' Luis Martinez contributed to this report.