Rescue teams were racing on Saturday to extricate workers trapped in hydropower tunnels after this week’s devastating floods in Nepal, even as the Nepal Army said its thermal drones were not detecting any signs of life.

There could be many reasons behind the lack of drone signals, including the thickness of the tunnel walls and the challenge of scanning a vast, rugged landscape radically altered by raging waters. But each passing day increases the likelihood that the over 900 workers who remain trapped will not make it out alive.

Two thermal drones used by the Nepal Army have “not detected any signs of life, but that could be because of the distance,” Raja Ram Basnet, an army spokesman, told The New York Times. “We are not sure how far the people might have been from the area where the drones were used.”

Enormous flash floods created on Wednesday by a falling slab of glacier along Nepal’s border with Tibet have left at least 675 dead, while more than 2,400 are still missing. After initially stating that around 1,000 people were unaccounted for, the Nepali government roughly doubled the count on Friday, after hydroelectric producers reported their tally of workers missing from projects in the region.

To get to those workers, rescue teams have to dig through dense mud and debris, then bore through thick tunnel doors or walls, or blast them open with explosives. Even once they’re in, the tunnels themselves are often filled with the sludge swept in by the flood. Photographs and footage released by the army showed rescuers dragging workers out of the tunnels’ caked-in mud after the entrances had been unblocked.

On Saturday afternoon, army personnel were trying to penetrate a tunnel at Trishuli 3A — one of five hydropower projects under construction in Nepal’s stretch of the Himalayas that were damaged by the flood — but had managed to enter by only about seven yards, Mr. Basnet said.

Army personnel also had to build a temporary helipad at the site so their helicopter could land, according to a person with direct knowledge of the operation who was not authorized to speak publicly.

Nepal has built a robust hydroelectric industry, harnessing the fast-flowing waters in the Himalayas to generate megawatts of electricity. Those projects, which require both manual and engineering work to construct tunnel systems deep into the earth, have also attracted foreign investments. Many hydropower projects were constructed along the Bhotekoshi and Trishuli Rivers, which bore the brunt of the flooding.

So far, 279 workers have been rescued from the tunnels, but 933 remain trapped, according to Nepal’s National Disaster Risk Reduction and Management Authority.

The army, which has been leading the search and rescue efforts, turned to the trapped workers more than a day after the catastrophe, according to army officials, industry lobbying groups and private rescuers. At some sites where hydroelectric power projects were under construction, army personnel and officials from the Nepal Electricity Authority started with survey trips on Thursday and returned the next morning with equipment to bore through the tunnels.

Some critics said the Nepali authorities could have taken a more methodical approach and moved faster, knowing that the difficult terrain and nature of the catastrophe would create additional challenges.

“If there was timely rescue from the areas, some people would have been alive,” said Bhim Gautam, chief executive of the Independent Power Producers’ Association, a lobbying group.

A group of mountaineering guides had been kept on standby since the day of the massive flood but wasn’t pulled into action until Saturday morning, said Ashish Gurung, a mountaineer who has climbed Mount Everest.

Mr. Gurung, who helped with rescue operations at Chilime, another hydropower plant, said his team used ropes and wooden planks to enter a tunnel there, which was filled with several feet of sludge. They made it about 130 yards before encountering a blockage, he said.

Mr. Gurung said they were in shock when they found a rotting body and a dismembered right foot, which they took to an army base where they could be placed in cold storage. “It would have been inhuman to leave the body behind,” he said.

Mr. Gautam, the power producers’ lobbyist, said his group had pushed the government to seek technical help from countries with more sophisticated technology to save time, especially since rescue efforts for trapped workers started in earnest only on Friday morning.

“We understand they must give priority to general people, but they must care about the life of people working in the hydropower sector,” Mr. Gautam said, adding: “There is no water, no food, there is no anything, there is no oxygen” in the tunnels. “It’s late, but we are hopeful.”

On Saturday, Nepal’s government said teams from India and China would provide technical assistance for the search. Late that night, Sudan Gurung, the home minister of Nepal, said on social media that rescuers had reached the crown of the Trishuli 3A tunnel.

“We have already started supplying air through that small hole,” he said, “and we will now begin excavation work and send the rescue team inside.”