Rescuers battled Wednesday to reach survivors of a deadly new earthquake in Nepal that triggered landslides and brought down buildings, as hundreds of troops searched for a missing US military helicopter.

Thousands of traumatised survivors spent the night outdoors, afraid to return to their houses after Tuesday's 7.3-magnitude quake hit, less than three weeks after the country was devastated by its deadliest quake in more than 80 years.

Dozens of people were killed in the latest disaster, bringing the overall death toll to more than 8,200 and compounding the difficulty of reaching far-flung mountain communities in desperate need of shelter, food and clean water.

The United Nations said it faced a "monumental challenge" to bring relief to victims, many of whom live in areas accessible only on foot or by helicopter, before the monsoon rains began.

UN resident coordinator Jamie McGoldrick said Tuesday's quake had exacerbated an already desperate situation as he delivered an urgent call for more funding.

"Damaged houses will be further damaged. Houses and schools and buildings that were spared in the first one were probably affected yesterday. Roads, we know, have been damaged," he said.

"It is extremely urgent that the donors realise that we are running against the monsoon that is coming."

- 'No hope left' -

The Nepal army mounted a huge ground and air search for a US Marine Corps helicopter that went missing on Tuesday during a disaster relief operation in the eastern district of Dolakha, near where the latest quake hit.

Spokesman Jagdish Pokharel said the army would keep searching until it found the chopper, which was carrying six US Marines and two Nepal army soldiers when it disappeared in a remote mountainous area.

"We have been searching for the missing helicopter since early morning but have not found it yet," he said.

"They came to help us here... (and) we will continue to search for them."

Police said 76 people had been confirmed dead and more than 2,000 injured in the new quake, which was centred 76 kilometres (47 miles) east of Kathmandu, and also killed 17 people in northern India.

"We had been focusing on relief distribution, but from yesterday our resources were deployed for rescue operations again," said Laxmi Prasad Dhakal, spokesman for the Nepal home ministry.

Dolakha and Sindhupalchowk, two of the districts worst affected by the original quake, bore the brunt of the damage caused by the fresh tremors.

The wounded -- many of them elderly -- continued to arrive Wednesday at a Norwegian Red Cross field hospital in Chautara in Sindhupalchowk, where shops remained closed and the streets empty of people.

Nurse Rannvig Anderson said the first patient had arrived at the hospital just 10 minutes after the quake, and staff had had no respite since.

"People have no hope left," 62-year-old shopkeeper Ripunath Nepal told AFP in Chautara. "We know that another quake is coming."

The Nepalese government has acknowledged that it was overwhelmed by the scale of the April 25 disaster, which destroyed nearly 300,000 homes and left many more too dangerous to live in.

Tuesday's quake was felt as far away as New Delhi, and caused buildings to collapse in Tibet in neighbouring China, killing at least one person there. A second tremor and more aftershocks also followed.

- Large-scale casualties -

There were several reports of landslides blocking roads in the worst-hit areas, making the task of getting relief to remote communities in the Himalayan country even more difficult.

"Although our personnel are already present in quake-hit areas, these blockages will make it difficult to transport relief materials," said police spokesman Kamal Singh Bam.

Many in Kathmandu had begun to return to their homes after weeks sleeping outdoors, but after Tuesday's strong quake and tremors, large numbers once again spent the night under canvas.

"Yesterday's quake shook us all, I couldn't imagine sleeping in our house with the kids. But we haven't been able to sleep in the tents either," said Kabita Maharjan, a 38-year-old mother of two young children.

"It was shaking all night, how could we? My kids were terrified. Who knows what will happen now."

Scientists said Tuesday's quake was part of a chain reaction set off by the larger one that struck on April 25 in Lamjung district west of Kathmandu.

"Large earthquakes are often followed by other quakes, sometimes as large as the initial one," said Carmen Solana, a volcanologist at Britain's University of Portsmouth.

"This is because the movement produced by the first quake adds extra stress on other faults and destabilises them," she told the London-based Science Media Centre.