Officials are struggling to locate 1,000 EU citizens who remain unaccounted for in Nepal, nearly a week after an earthquake that killed more than 6,000 people.
An EU official said most are thought to have been trekking near Mount Everest or in the remote Langtang mountains.
Many are hoped to be alive in isolated areas cut off by the quake.
Nepal has called for more foreign help, particularly aircraft, as survivors grow desperate for humanitarian aid.
Nepalese authorities say the death toll from Saturday’s 7.8-magnitude quake could rise to 10,000, with uncertainty still surrounding the fate of thousands of people in remote mountain communities.
Nearly 14,000 people were injured in the disaster. Relief and rescue teams have reached most areas but many people remain in urgent need.
At the scene – Sanjoy Majumder, BBC News, Kathmandu
At Kathmandu’s historic Durbar Square, soldiers and volunteers form human chains to remove the debris, brick by brick.
The bricks come from temples and other historic buildings levelled by the earthquake. Many are very old and are being stored so that they can be used to rebuild these ancient sites.
The soldiers are joined by aid workers – but also tourists. One French visitor said she “just wanted to help”. But it’s an ad hoc approach which characterises the entire relief operation.
I met rescue and medical teams from France and China. After wandering around they left. “We don’t know what we are supposed to be doing,” one French rescue worker told me.
Their services are required in the remote villages where many are in urgent need of assistance – but they are stuck here in the capital because no-one is telling them what to do.
The quake struck during peak trekking season in a country popular with mountaineers. Twelve EU citizens are known to have died so far. Some of them were killed by avalanches, triggered by the quake, that struck base camp below Mount Everest.
The EU envoy to Nepal, Rensje Teerink, said the authorities did not know the whereabouts of some 1,000 other EU citizens.
“They are missing but we don’t know what their status is,” she told reporters in the capital, Kathmandu.
Another EU official, speaking to the AFP news agency on condition of anonymity, said most of the missing were likely to be found safe.
Many backpackers do not register with their embassies when they arrive in Nepal, which has made it harder to trace them, Reuters news agency reports.
Landslides and poor weather have hampered efforts to reach isolated districts.
“Almost every house in my village is destroyed, and 20 people died,” Kumar Ghorasainee told the AFP news agency amid the ruins of his hometown of Melamchi in Sindhupalchok, north-east of Kathmandu.
The region has been identified by the Red Cross as one of the worst-affected areas.
“We lost our cattle and our sheep,” the 33-year-old English teacher said. “No-one has come to help us – the cars and the aid trucks just drive by… How will we manage now?”
There are only about 20 helicopters available for the rescue and relief operations taking place in a region that stretches hundreds of kilometres.
Fights have broken out with victims demanding to be evacuated and not enough space on board helicopters, the BBC’s Sanjoy Majumder reports.
China is expected to send more, but still more are needed – along with road vehicles to carry relief supplies.
“Inaccessibility to some remote areas, the lack of helicopters, poor communication and security concerns remain the main challenges in delivering relief,” said Farhan Haq, a United Nations spokesman.
The UN on Wednesday launched a “flash appeal” for $415m (£270m) to assist Nepal over the next three months – but on Friday it told the BBC it had so far received only $5.8m – 1.4% – in confirmed funding.
There has been growing anger at the government’s response to the disaster, with a number of protests breaking out.
In the capital Kathmandu, shortages of food and water have forced thousands of workers to board buses and flee to their home towns and villages.
A lack of buses and the slow distribution of aid have led to clashes between protesters and riot police.
The tent cities which sprang up around Kathmandu in the days following the quake have almost gone, reports say, as the fear of aftershocks subsides.
But the stench of rotting bodies in the rubble has driven officials to order that unclaimed bodies be cremated immediately after they are recovered, Reuters news agency reported.
The UN has also warned of the challenges facing Nepal’s farming community, which comprises up to two-thirds of the country’s 27 million people.
It says that the quake destroyed seed stocks for the mid-May rice sowing season, as well as grains kept dry in stone storage huts that have now been razed to the ground.
If farmers miss this month’s planting season, they will be unable to harvest rice – Nepal’s staple food – until late 2016, the UN says.