"The government has been continuing efforts... to rescue people trapped inside the tunnel of the Trishuli 3A Hydropower Project in Rasuwa," the office of Nepal's prime minister said in a statement.

"As part of these efforts, work to clear the tunnel entrance has been intensified since early Saturday morning," it said.

Those trapped are among 933 missing people who work on hydropower projects on the rivers where a wall of water swept down on Wednesday morning, according to the National Disaster Risk Reduction and Management Authority.

The agency said on Thursday that the army had already rescued 350 people under "extremely risky" conditions, but that more than 100 remained trapped.

Initial reports suggested the 100 trapped were in a single location, but officials said that rescue efforts were also underway at other hydropower projects.

Minister of Energy Biraj Bhakta Shrestha said operations were also ongoing at the Trishuli 3B site "despite the adverse weather and extremely challenging circumstances".

Photographs shared by the disaster office showed workers wading through churned mud and navigating vast boulders left by the floods.

Foreign Minister Shisir Khanal told reporters in Kathmandu that an 11-member Indian expert tunnel rescue team had begun work, and a team from China was "expected to arrive very soon".

India has extensive experience with difficult Himalayan tunnel rescues, including teams of "rat-hole" miners trained in being able to crawl into narrow spaces.

Khanal said conditions were tough and appealed for assistance, including heavy-lift drones and technology to "help us locate people" who are trapped.

A Nepal Army statement said on Saturday that rescuers were "using equipment including drilling machines in an effort to locate the outer section of the tunnel".

The US Geological Survey said the disaster on the China-Nepal border on Wednesday was caused by a glacial collapse that created a torrent of debris, with nearly 3,000 people missing and 633 confirmed dead.