Hungry and desperate villagers have rushed towards relief helicopters in remote areas of Nepal, begging to be airlifted to safety, four days after a devastating earthquake killed more than 5000 people.

“The ground keeps shaking, even this morning it did. Every time it feels like we will be swallowed, that we will die now. I want to get out of here!” said Sita Gurung, 24, whose home had been wrecked.

As the Himalayan nation’s Prime Minister Sushil Koirala said getting help to remote areas was a “major challenge”, aid finally began reaching areas that had to fend for themselves since Saturday’s 7.8-magnitude quake.

In a televised address late Tuesday, Koirala declared three days of national mourning for the 5057 people known to have perished in Nepal alone.

More than 100 people died in neighbouring countries such as India and China.

Around 8000 people had been injured while the United Nations estimated that eight million people had been affected.

Melbourne woman Renu Fotedar was among the 18 climbers who were killed at Mount Everest base camp when an avalanche triggered by the quake flattened everything in its path.

Countries far and wide have joined the relief effort in what is one of Asia’s poorest countries, with neighbouring India playing a leading role.

In Gorkha, one of the worst-hit districts, terrified residents ran with outstretched arms towards an Indian army helicopter to plead for food and water.

An AFP journalist on board saw scores of houses across several villages in the district turned into twisted mounds of wood and corrugated tin roofs.

“We haven’t had any food here since the earthquake,” Gurung told AFP.

Koirala told an emergency all-party meeting the government was sending desperately needed tents, water and food supplies to those in need.

But he said authorities were overwhelmed by appeals for help from remote Himalayan villages.

“Appeals for rescues are coming in from everywhere,” a statement from Koirala’s office quoted him as saying.

“But we have been unable to initiate rescue efforts in many areas at the same time due to lack of equipment and rescue experts.”

In a sign of how difficult conditions are, Nepalese official Uddav Prasad Bhattarai said 250 people were feared missing after an avalanche Tuesday on the popular Langtang trekking route.

With fears rising of food and water shortages, Nepalis were rushing to stores and petrol stations to stock up on essential supplies in the capital Kathmandu.

Nepal has declared a state of emergency after the disaster, its deadliest in more than 80 years.

Families who work in Kathmandu were packing onto buses – some even sitting on the roofs – in an exodus from the city.

Those who remained in the capital were sleeping outdoors in tents in parks and other open spaces. Many had lost their houses, others were too terrified to return home after several powerful aftershocks.

With just plastic sheets to protect them from the elements, many were desperate for aid and information on what to do next.

“We’ve been staying here for three days, living under canvas. We’re counting every bite we eat, every drop we drink,” said 28-year-old housewife Rama Shrestha, who was camping out with her five-year-old son.

Hospitals have been overwhelmed, with morgues overflowing and medics working flat out to cope with an endless stream of victims suffering trauma or multiple fractures.

The United Nations said it was releasing $US15 million ($A19.09 million) from its emergency fund to help relief efforts while the World Food Program said it aimed to get food aid to 1.4 million people over the next three months.

Australia said it was raising its level of aid to $A6 million and sending a military plane to bring in relief supplies and evacuate stranded citizens.

But lack of space at the only international airport was hampering efforts to bring in relief by air.