Nepal’s rescue teams, aided by Chinese and Indian experts, are racing to save more than 900 workers believed to be trapped underground at hydropower projects hit by devastating flash floods.

Nepali officials said on Monday that the focus was on reaching the workers in about a dozen hydropower project tunnels blocked by mud and rock debris.

The unprecedented deluge of ice, rock, mud and debris on the Nepal-Tibet border on Wednesday – caused by a glacier collapse – has killed 903 people with 4,247 missing in Nepal, its disaster authority said.

The flood also wreaked havoc across the border in China’s Tibet region, where at least 16 people died and 546 are missing, said Chinese authorities.

‘Big challenge’

Pictures and videos shared by rescue teams in Nepal showed earth-moving equipment digging and shovelling mud and rocks under lights in the dark of the night as workers watched. Soldiers used torchlights in a big pit they had created to look for space to get into one tunnel.

“A large amount of debris has been deposited and that is posing us a big challenge to open the tunnels,” Nepal army spokesperson, Raja Ram Basnet, told the Reuters news agency.

“Our main focus is on opening the tunnels.”

Hundreds of bodies, many unidentified, have been buried in shallow graves as they began decomposing in the humid weather after being swept down the mountains by the torrent.

At a hospital in the capital of Kathmandu, morgues have reached capacity and families are flocking to other hospitals in hopes of finding missing loved ones, reported Al Jazeera’s Minelle Fernandez.

In addition to the 933 missing hydropower workers, some 500 workers from the Truck and Container Association are also missing, said Fernandez.

The Red Cross has estimated that more than 90,000 people are likely to have been affected by the disaster, which China has linked to the effects of climate change.

Prime Minister Balendra Shah said the flood was an “unprecedented and devastating natural disaster” and it remained challenging to compile a complete assessment of the damage.

“Despite the adverse weather, difficult terrain and continuing risks, we are moving forward with determination to save the lives of our citizens,” he said in a Facebook post late on Sunday, adding that nearly 21,000 security personnel were involved in rescue operations, besides civilian workers and volunteers.

While accessibility has been restored on many roads and highways, almost “40 kilometres of roads have been absolutely swept away”, Al Jazeera’s Fernandez said.

“So there are challenges on the ground in getting this aid to communities, but the authorities, as well as the agencies and their networks are working their way through it.”

China sends experts, aid

China’s state-run CCTV said the government has mobilised more than 2,100 rescue workers and allocated at least 220 million yuan ($33mn) to support relief efforts. Equipment and supplies have been airdropped to frontline rescuers, while Chinese soldiers used life-detection equipment to look for the missing.

Beijing said 261 foreign nationals from 23 countries were unaccounted for in Tibet.

“President Xi Jinping, according to state media, has called for all out efforts to rescue those missing in the mudslide, and this is the second time that the president has made such a call,” said Al Jazeera’s Katrina Yu, reporting from Beijing.

“So this seems to be the Chinese leader’s effort to call on people, not only to continue their efforts, but redouble their efforts even as things are looking grimmer and grimmer.”

China has sent nine tunnel rescue experts to Nepal and shared satellite information with Nepalese authorities, said Yu.

Although Nepal has said it does not need foreign help in general search and rescue efforts, it has accepted help from giant neighbours India and China for their expertise in tunnel rescue.

Rescue operations have been suspended several times since Friday because of bad weather and concerns that a lake, formed across the Nepal-China border by the disaster, could trigger new flooding after it began overflowing into Nepal’s rivers.

The weather was good on Monday and rescue work continued to pick up pace, said Narendra Pariyar, the district administrator of Rasuwa, one of the two hard-hit districts.