Twenty-six people were confirmed dead on Monday and more than 170 others were missing after a devastating flash flood in India thought to have been caused by a chunk of glacier breaking off.

The resulting wall of water and debris barrelled down a tight valley in India's Himalayan north on Sunday morning, destroying bridges, roads and hitting two hydroelectric power plants.

Uttarakhand Director General of Police Ashok Kumar said late Monday that 26 bodies had been recovered, and 171 people were still unaccounted for.

Most of those missing were workers at the two power plants, with some trapped in a U-shaped tunnel filled with mud and rocks when the flooding hit.

"If this incident happened in the evening, after work hours, the situation wouldn't have been this bad as labourers and workers in and around the sites would have been at home," Uttarakhand Chief Minister Trivendra Singh Rawat told reporters.

Twelve people were rescued from one side of the tunnel on Sunday but another 34 were still trapped at the other end, the Indo Tibetan Border Police's Banudutt Nair, who is in charge of the rescue operation, told AFP.

With the main road washed away, paramilitary rescuers were forced to scale down a hillside on ropes to reach the entrance. Emergency workers were using heavy machinery to remove tonnes of rocks.

"Approximately 80 metres (260 feet) inside the tunnel is cleared and accessible," said Vivek Kumar Pandey, another disaster official.

Around 1,000 rescuers -- including from the military, police and national disaster personnel -- resumed their search operation at first light on Monday.

Three excavators had reached the site, with one removing the slush and depositing it in the nearby river, Nair said, adding that the semi-liquid debris was estimated to be around 180 metres deep.

Rescuers believed there were air pockets in the tunnel, Nair added.

Grim reminder

Scores of social media users captured the disaster, with footage showing water tearing through the narrow valley with terrifying force.

"We were 300 metres inside the tunnel working. Suddenly there was whistling and shouting telling us to get out," said survivor Rajesh Kumar, 28.

"We started running out but the water gushed in. It was like scenes from a Hollywood movie. We thought we wouldn't make it," he told AFP.

Deepak Kumar, whose brother Bharminer is trapped inside the tunnel, said six of the workers had come from his village in Lakhimpur Kheri in the state of Uttar Pradesh.

"Out of that, one survived and he informed us, and the moment that we heard that our brother is trapped inside the tunnel, we jumped into the car" and drove around 400 kilometres overnight, Kumar told AFP outside the tunnel.

Authorities said initially that the cause was a chunk of glacier breaking off into a river, but the trigger may instead have been a phenomenon called a glacial lake outburst flood (GLOF).

This is when the boundaries of a glacial lake -- formed when a glacier retreats -- are breached, releasing large amounts of water downstream. 

It is possible that this in turn was caused by an avalanche. The incident may also have been triggered by water pockets inside a glacier bursting.

Glaciers in the region have been shrinking rapidly in recent years because of global warming, but experts say that the construction of hydroelectric plants could also be a factor.

Floods in 2013 killed 6,000 people and led to calls for a review of projects in Uttarakhand, a state of 10 million people bordering Tibet and Nepal.

Vimlendhu Jha, the founder of Swechha, an environmental NGO, said the disaster was a "grim reminder" of the effects of climate change and the "haphazard development of roads, railways and power plants in ecologically sensitive areas".

A major study in 2019 said that two-thirds of Himalayan glaciers, the world's "Third Pole", could melt by 2100 if global emissions are not sharply reduced.

Glaciers in the region are a critical source of water for hundreds of millions of people, feeding many of the world's most important river systems.