More than 1,200 people have been killed and some 5,600 are missing on both sides of the China-Nepal border since the catastrophic glacial mountain collapse on August 26, according to tolls by both nations on Thursday.

"It is just incredible -- the magnitude of the disaster, the magnitude of the destruction," UN Resident Coordinator Lila Pieters Yahia told reporters in Kathmandu.

More than 800 foreigners from at least 34 countries are among those missing, including tourists and pilgrims, in both Nepal and Tibet in China.

In Kathmandu, families of some of the missing foreigners said they were combing hospitals and morgues for any information.

"I feel desperate, it's been so many days," said 53-year-old US citizen Sanjeev Rai who flew to Nepal to look for his wife, one of around 75 Americans missing.

In China, photographs of missing people are being shared on social media by relatives hoping for news.

"Hurry up and come back, your parents miss you a lot, we need you," one woman wrote on a popular platform, posting an image of her relative, a police officer, and his wife.

Beijing keeps tight control over media access and information, and foreign journalists are restricted from reporting in Tibet.

- 'Very, very difficult' -

Floods tore through mountain districts, destroying roads, bridges and hydropower dams.

Communities near the epicentre remain cut off, with the UN working alongside Nepal's government to get emergency supplies into devastated areas.

"Access has been very, very, very difficult," Pieters Yahia said, noting that UN agencies had delivered water and and sanitation kits, as well as food and medicine.

"Those are the most essential, particularly for the people who are in the epicentre."

Foreign Minister Shisir Khanal, in a briefing to diplomats in Kathmandu on Thursday, called it a "Himalayan tsunami".

"I can say from first-hand observation that the reality on the ground is far more severe than images can convey," he said. "This is not an ordinary flood".

Foreign ministry spokesman Lok Bahadur Poudel Chhetri said Nepal needed specialised international assistance, including heavy-lift drones and temporary bridges, as well as body bags and DNA testing kits to help identify the dead.

He said Nepal also needed a mobile air traffic control system and broadband seismometers to monitor ground vibrations.

More than 21,000 Nepali security personnel are working on search, rescue and relief operations.

Nepali rescuers, aided by experts from India, China and South Korea, continue searching for survivors feared trapped inside hydropower tunnels, but teams have repeatedly hit dead ends, and no one has been confirmed alive.

The energy ministry said a committee will be set up to coordinate with rescuers and provide necessary assistance to the families of missing hydropower workers.

At a relief camp in Nepal's Nuwakot district, an AFP journalist saw flood survivors waiting for their names to be called before collecting bags of rice, instant noodles, cooking oil and biscuits.

Pieters Yahia said the UN was discussing with the government "how to use helicopters and cargo drones to be able to deliver aid".

- Monitoring and data sharing -

The UN may also turn to Nepal's porters, accustomed to carrying heavy loads through some of the world's toughest mountain terrain during trekking and climbing expeditions, to "bring the aid as soon as possible", the resident coordinator said.

Pieters Yahia warned that gaps in assistance were inevitable given the scale of the catastrophe.

"The magnitude of the disaster is so big that there will be always gaps," she said.

Tibetan advocacy groups in exile have questioned China's account of the disaster, but Beijing insists that the "release of information has been open, transparent and timely".

The disaster highlights the need for better long-term monitoring, data sharing and coordination across the Himalayas, experts say.

"What we really need is a dedicated mechanism... which works around the clock," Saswata Sanyal with the Kathmandu-based International Centre for Integrated Mountain Development told AFP.

Nepal's disaster authority said Thursday that the number of bodies recovered was 1,259, with 5,083 people missing -- a sharp rise from the previous figure issued earlier in the day.

In China, state media say 21 people had bene killed, and 541 people are missing.

That takes the overall toll to 1,280 people, with 5,624 missing.