About 500 feared stuck in hydropower tunnels in Nepal

Rescuers are racing to free hundreds of workers believed to be trapped in tunnels at hydropower sites that were buried in mud by catastrophic flooding on the Nepal-China border as the death toll climbs and increasingly desperate families searched for missing loved ones.

The combined deaths in Nepal and China surpassed 950, with more than 4400 people - including 43 Australians - missing.

A private-sector association representing power companies said at least 900 workers were unaccounted for at 12 hydropower projects damaged by the flash floods in Nepal's Rasuwa and Nuwakot districts.

At least one expert said that some workers could have access to oxygen but others warned that the chances of finding survivors were diminishing.

As the rescue efforts intensified, four bodies were recovered from three mud-filled tunnels, Nepal's army said.

Some workers managed to call for help when the disaster struck last Wednesday, and others who managed to flee reported to authorities and relatives that many of their colleagues were still in the tunnels, said Mohan Kumar Dangi, president of the Independent Power Producers' Association.

It was unclear whether rescuers have had any contact with the trapped workers since the initial calls for help.

Six days after the disaster, fears were growing that some might have suffocated, Dangi said.

Rescue efforts were focused on people trapped in the gigantic tunnels built for controlling the flow of water.

"These tunnels are big enough to drive trucks through," said Jakob Steiner, a geoscientist at the University of Graz in Austria based in Dhaka, Bangladesh.

"And from what we understand, they have managed to get oxygen to some of those stuck inside and are trying to rescue them now. It's a race for time, though, as people need food and water."

In the Upper Trishuli-1 Hydropower Project, in the hard-hit Rasuwa district, at least 576 people were missing, Dangi said, adding that about 300 of them were believed to be trapped inside the project's tunnel.

At the Trishuli-3 Hydropower Project, crews were working to descend about 15 metres into a tunnel in an effort to reach those believed trapped inside, according to Nepali army spokesman Brigadir General Raja Ram Basnet.

Workers shovelling mud from a flood-ravaged power station in Dhading, Nepal, said the station is a key connection for providing power to more than 20,000 people in the region.