The death toll from an earthquake that ripped through Nepal will climb "much higher", the government warned Sunday, as relief teams raced to reach survivors of the disaster that has already claimed more than 7,000 lives.

Finance Minister Ram Sharan Mahat said the toll will jump once teams reached far-flung villages feared flattened in the worst quake to hit the impoverished Himalayan nation in more than 80 years.

"There are still villages where we know that all houses have been destroyed, but have not yet been able to reach," Mahat said in a statement.

"The aftershocks have not receded and we expect the final casualty numbers to climb much higher," the minister said, as he appealed for hundreds of millions of dollars in foreign donations to help rebuild the devastated country.

The 7.8-magnitude quake wreaked a trail of death and destruction when it erupted around midday eight days ago, reducing much of the capital Kathmandu to rubble and triggering a deadly avalanche on Mount Everest.

After the government ruled out finding more people buried alive in the ruins of Kathmandu on Saturday, the focus has shifted to delivering food, tents and other relief to thousands in remote areas.

In the hardest-hit districts of Gorkha and Sindhupalchowk, almost 90 percent of the mostly stone and mud homes have been destroyed, the United Nations said in its latest situation report.

"Our priority now is really to try to reach those people (in remote areas), get immediate assistance to them," the UN's head of humanitarian affairs Valerie Amos said.

But Amos said on Saturday that she was worried that the tonnes of foreign aid pouring into Nepal was being held up by red tape.

"I was extremely concerned to hear reports that customs was taking such a long time," Amos told AFP, saying she had asked Prime Minister Sushil Koirala to speed up customs clearance for aid materials.

"He has undertaken to ensure that happens, so I hope that from now we will see an improvement in those administrative issues."

Planes loaded with relief supplies from around the world were pouring into landlocked Nepal, but there have been numerous reports of many getting stuck at Kathmandu's small international airport, and customs officials stopping trucks filled with aid from crossing into the country from neighbouring India.

The airport has been closed to heavy planes because of concerns about the condition of the single runway after the quake and a series of strong aftershocks, said Birendra Prasad Shrestha, manager of the Tribhuvan International Airport.

"We have issued a notice saying that aircraft with a total weight exceeding 196 tonnes will not be allowed to land at Kathmandu airport," Shrestha told AFP.

"There are no visible cracks in the runway but there have been so many tremors recently that we have to take precautions -- we don't know what's happening below the surface.

"This runway is the only lifeline for Kathmandu -- if it goes, everything goes."

- Bodies of foreigners found -

The death toll from the disaster has hit 7,040, according to the Emergency Operations Centre, with more than 14,000 injured. More than 100 were also killed in India and China.

Fifty one bodies, including those of six foreigners, have been found in recent days in the popular Langtang trekking region hit by a quake-triggered avalanche.

"We estimate that about 100 foreigners might still be missing in the area," senior local official Uddav Prasad Bhattarai also said.

"Our priority was to get the survivors out. We rescued over 350 people, about a half of them were tourists or guides," he told AFP in Rasuwa district that includes Langtang, north of Kathmandu.

The Red Cross has warned of "total devastation" in far-flung places, where whole villages have been destroyed and survivors are sleeping in the open.

The UN's situation report says teams that have arrived in Gorkha have discovered a "dire need for shelter, particularly tents and blankets".

"Access to some remote villages remains a key challenge as many landing zones are unsafe due to debris, altitude and current weather conditions," the report also says.

"Road access is limited. Some remote villages can only be accessed by helicopters."

In Kathmandu, tens of thousands of survivors have been living out in the open since the quake, having either lost their homes or fearful that aftershocks could bring teetering buildings to the ground.

"We are not living in this tent out of choice. We are here because we have nowhere to go," said Dhiraj Thakur who has been camped out since the quake in Tundikhel Maidan, an open area in the centre of the city.

Although multiple teams of rescuers from more than 20 countries have been using sniffer dogs to find survivors in the rubble, no one has been pulled out alive in the capital since Thursday evening.