KATHMANDU, Nepal -- Thousands of people fearing aftershocks continued to sleep outdoors as authorities tried to reach survivors of Nepal's most recent earthquake cut off by blocked roads in isolated villages.

There was a shortage of tarpaulin and tents in the Nepalese capital and elsewhere and people were even using cardboard boxes as their temporary shelter after this Himalayan nation suffered through its second major quake on Tuesday in less than three weeks.

"We have nowhere to go. This is our home for now. We had just moved back into our rented rooms and again the earthquakes are back," Raj Kumar, a carpenter who was sharing a small tent with two other families, said Thursday.

A search also continued Thursday for a U.S. Marine Corps helicopter carrying six Marines and two Nepalese soldiers. It went missing Tuesday while delivering aid in the country's northeast, U.S. officials said. There have been no indications it crashed.

The magnitude-7.3 earthquake shook the impoverished country Tuesday, killing at least 91 people and injuring more than 2,300, just as it was beginning to rebuild from a devastating April 25 earthquake.

"I just don't feel safe anymore. I get scared even when cars or trucks drive past us and the ground shakes. We are all terrified. Now I am afraid that we might all get sick staying out in the open space, cramped into these tents and not getting enough water," Srijana Sharma, a house wife, said in Kathmandu.

The most recent quake hit hardest in deeply rural parts of the Himalayan foothills, hammering many villages reached only by hiking trails and causing road-blocking landslides.

"Damaged houses were further damaged or destroyed. Houses and schools building spared before were affected ... roads were damaged," said Jamie McGoldrick, a top U.N. official in Nepal.

Among 14 quake-hit districts, some are barely accessible, and a large part of the affected population could not be reached easily because of damaged roads.

"Some are even difficult to reach by helicopter. We are facing monumental challenges here to support the government in these districts," McGoldrick said.

On Wednesday, officials with bullhorns walked through the worst-damaged streets of Chautara, a small town northeast of Kathmandu, calling for people to leave buildings in danger of collapsing after Tuesday's quake.

"There is danger!" they said over the bullhorns. "Leave the buildings!"

Chautara, a foothills town, became a hub for rescuers and humanitarian aid after the first earthquake.

People salvaged whatever they could from their toppled homes. Most houses appeared to be damaged; some were levelled. Others tilted and rested on adjacent homes.

"We were in the shop. All of a sudden the building shook. I jumped out of the store and the next second it fell down. It was already tilted by last month's earthquake. I watched it just slide and fall on its side," said Devi Acharya, a convenience store owner.

Officials said that army helicopters had been searching through the Sunkhani area, nearly 80 kilometres (50 miles) northeast of Kathmandu, for the missing U.S. helicopter.

Tuesday's quake also killed 16 people in northern India and one person in Tibet.

The magnitude-7.8 earthquake that hit April 25 killed more than 8,150 people and flattened entire villages, leaving hundreds of thousands homeless in the country's worst quake since 1934.

The U.S. Geological Survey said Tuesday's earthquake was the largest aftershock of the April quake. But it was significantly less powerful and occurred deeper in the Earth.

The first quake also drove many people to leave damaged homes, which were empty when the new quake caused more damage and collapses.

On Wednesday, McGoldrick said the U.N. had revised its donor appeal to call for $423 million. The response to the earlier appeal of $415 million has been low, with about 15 per cent of the sum received.

- Associated Press writers Bernat Armangue in Chautara, Tim Sullivan in New Delhi and Lolita C. Baldor in Washington contributed to this report.