A desperate race is under way to reach hundreds of workers feared trapped deep inside hydropower tunnels in Nepal, as rescuers cling to one of the last major hopes of finding survivors a week after catastrophic flooding devastated the Himalayan region.

At least 1204 people have now been confirmed dead and more than 4200 remain missing after the disaster, which was triggered by the collapse of a glacier near Nepal’s border with China.

With the likelihood of finding people alive elsewhere rapidly diminishing, attention has turned to a network of tunnels beneath hydroelectric projects where workers may have escaped the initial torrent.

Authorities believe about 900 missing workers could be inside the tunnels, some of which contain chambers with air supplies as well as access to food and water.

“We are very confident that there is life in there because ... there is food and water in the building, that was likely built for communication purposes and as a resting place,” Nepal Electricity Authority board member Amshu Kiran Shahi told Reuters.

Major rescue operations are under way or being planned across four hydropower projects, with two sites where about 90 people are missing considered particularly promising because rescuers believe air remains inside.

At the Trishuli-3A project, officials believe a dry chamber may have survived the flooding and oxygen could still be flowing through the tunnel. Pipes are also being used to pump oxygen inside as teams attempt to reach those potentially trapped.

Specialist rescue crews from India, China and the United States have joined the operation.

But beyond the tunnels, authorities acknowledged the window for finding survivors was narrowing.

“The hope of finding survivors is decreasing,” senior National Emergency Operation Center official Phanindra Paudel said.

“We pray and are hopeful. Hope is hope.”

Nepalese Prime Minister Balendra Shah vowed rescue teams would continue searching despite the increasingly grim outlook.

“Our priority remains clear: To continue our search and rescue operations, find those who are still missing and continue supporting everyone affected,” he said.

Across the border, China has increased its death toll to 21, with another 541 people still missing.

Heavy machinery has begun reaching some of the hardest-hit areas after Beijing fully reopened the road to the Gyirong border crossing, which had been obliterated by the enormous wall of water and debris.

Footage from Chinese state broadcaster CCTV showed excavators, search dogs, rescue crews and life-detection equipment working through a narrow mountain gorge, where rain, unstable cliffs and fast-moving water have complicated the operation.

Teams are conducting a “deep search” around the remains of inspection buildings, parking areas and dormitories in the flattened disaster zone.

Among those listed as missing by Chinese authorities are 104 Nepalese citizens, 49 Indians and 33 Latvians believed to have been travelling in two tourist groups. Many of the missing Indians had been visiting Lake Mansarovar and Mount Kailash, sacred pilgrimage destinations for Hindus.

Beijing is also sending additional DNA identification specialists and emergency equipment to Nepal, including temporary Bailey bridges, mortuary refrigerators and communications equipment.

Back in Nepal, survivors have begun picking through the ruins of towns and villages in an attempt to recover whatever belongings remain.

“This was one of the major disasters experienced by human civilisation,” Mr Shah said.

“As soon as information was received that the flood was approaching, our security personnel were deployed to warn local residents. Some of those personnel themselves went missing.”

- With AP