The joyous rescue of two survivors of last weekend's Nepal earthquake added new urgency Friday to the search for signs of life in the ruins of Kathmandu as the Red Cross warned of "total devastation" in remote areas.

Six days on from the 7.8-magnitude quake, authorities put the number of dead in Nepal at more than 6,200 while around 100 more were killed in neighbouring India and China.

But the full extent of the destruction wrought by Nepal's deadliest earthquake in more than 80 years was still only just emerging as relief workers struggled to reach remote regions of the vast Himalayan nation.

Rescuers from more than 20 countries have been taking part in the search for survivors in the rubble-strewn capital Kathmandu as well as far-flung rural areas near the epicentre.

After hopes had begun to dwindle that anyone else would be found alive, the rescue effort received a double shot in the arm on Thursday when a 15-year-old was pulled to safety from inside a collapsed guesthouse before a woman in her 30s was rescued nearby late at night.

- 'Born again' -

After spending 10 hours trying to free Krishna Devi Khadka, the multinational team of rescuers greeted her emergence from the rubble with tearful whoops of joy.

"She was injured but she was conscious and talking," a Nepal army major told an AFP reporter at the scene.

"It is as though she had been born again."

The earlier rescue of 15-year-old Pemba Tamang, who told AFP that he stayed alive by eating a jar of ghee (clarified butter), was hailed as a miracle by medics who said he had suffered no more than cuts and bruises.

The rescues in Kathmandu offered a rare respite from the grim reports piling in from other parts of the country.

The Red Cross warned that nearly all homes had been wiped out in some towns and villages near the epicentre and was "extremely concerned" about the welfare of hundreds of thousands of people in Nepal.

"Six Red Cross assessment teams are reporting that some towns and villages in the worst-affected districts close to the epicentre have suffered almost total devastation," it said in a statement.

The destruction appeared particularly dire in the Sindhupalchowk region, a mountainous area northeast of Kathmandu, which was becoming a major focus of international relief efforts.

"One of our teams that returned from Chautara in Sindhupalchowk district reported that 90 percent of the homes are destroyed," said Jagan Chapagain, head of IFRC's Asia Pacific division.

"The hospital has collapsed, and people are digging through the rubble with their hands in the hope that they might find family members who are still alive."

Locals in Gorkha district, another of the worst-hit areas, said their misery was being compounded by terrifying aftershocks that were still being felt more than five days after the quake.

- 'Ground still shakes' -

"The ground still shakes a little every day. We don't know when we are alive, when we are dead," Gopal Gurung told an AFP reporter in the village of Laprak as an Indian military team delivered supplies.

"We are not protected, it is raining all the time, (we don't) know what can happen. Scared, people scared now," he added in broken English.

The latest toll from Nepal's National Emergency Operations Centre put the number of dead at 6,204, adding that a further 13,932 people were injured.

With so many families in need, the Nepal Red Cross Society said it had almost exhausted its relief stocks which were sufficient for 19,000 families.

The UN's food agency issued an appeal for $8 million in donations that it said were urgently needed to help farmers and avert a crisis in the food supply.

A primary concern is making sure farmers do not miss the planting season for rice, Nepal's staple food, which is expected to begin in late May, the United Nations' Food and Agricultural Organisation (FAO).

Failure to plant in time would leave farmers without a crop to harvest until late 2016 in a country where around two-thirds of people rely on agriculture for their livelihood.

"There is a critical window of opportunity to help crop producers plant in time to have a rice harvest this year," said Somsak Pipoppinyo, FAO representative in Nepal.

The Nepalese government, which has admitted being overwhelmed by the disaster, said it hoped more relief supplies could be flown in to those in desperate need of food and shelter.

"Rescue efforts were intensely moving forward, but now relief distribution is also gaining pace to reach all affected areas," home ministry spokesman Laxmi Prasad Dhakal told AFP.

"Helicopters are being deployed to reach geographically difficult locations."