Rescuers have pulled a 15-year-old boy alive from the rubble of Nepal's earthquake as the death toll from the disaster climbed to nearly 6000.

The rescue of Pemba Tamang on Thursday was hailed as a miracle and greeted with cheers from crowds of bystanders who watched the drama unfold at a ruined guesthouse in Kathmandu.

But the recovery of another teenager's body from the same ruins only minutes later underlined how the prospects of finding further survivors of Saturday's 7.8-magnitude quake are becoming more remote.

Caked in dust, Pemba was fitted with a neck brace and raced to a field hospital where he was found to have only minor cuts and bruises.

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"I never thought I would make it out alive," the teenager told AFP at the Israeli military-run facility.

Pemba, who worked at the guesthouse as a bellboy, said he had been eating lunch next to reception when the ground started shaking.

"I tried to run but ... something fell on my head and I lost consciousness - I've no idea for how long," he said.

"When I came round, I was trapped under the debris and there was total darkness," he added.

"I heard other people's voices screaming out for help around me ... but I felt helpless."

Asked if he had had anything to eat while he was trapped, Pemba said he had come across a jar of ghee (clarified butter) in the dark.

"I don't know where it came from," he added.

Libby Weiss, a spokeswoman at the Israeli field hospital, said Pemba was doing "remarkably well", confirming he did not have any major injuries.

"He was under the rubble for 120 hours and it is certainly the longest we have heard anybody of being under the rubble and surviving," she told AFP.

"I don't have any logical explanation. It is miraculous. It is a wonderful thing to see in all this destruction."

Launching an appeal for $US415 million ($A518 million) in aid, the UN said it would take a marathon effort to help the people of one of Asia's poorest countries.

The UN's World Health Organization said it had received reports that about 1400 had been killed in the Sindhupalchowk region, a mountainous region northeast of Kathmandu which was becoming a major focus of international relief efforts.

Nepalese and Indian military helicopters have also been flying in aid to remote areas of Gorkha district, another badly-hit region, which otherwise takes up to 12 days to reach by foot, the WHO said.

"But the needs remain great, with helicopters unable to reach some communities due to poor weather and steep hillsides," said a statement. "Reaching these communities and providing health support is essential."

The latest official toll put the number of dead at 5844 and more than 10,000 are known to have been injured.

More than 100 people were also killed in neighbouring countries such as India and China.