Emergency workers in India raced against time today to dig through mud and rocks choking a Himalayan tunnel after a devastating flash flood - thought to have been caused by a chunk of glacier breaking off - killed at least 26 people.

More than 170 others were listed as missing after a wall of water and debris barrelled down a tight valley in India's north early yesterday, destroying bridges, roads and hitting two hydroelectric power plants.

Uttarakhand Director General of Police Ashok Kumar revealed that 26 bodies had been recovered, and 171 people remained unaccounted for.

Most of those missing were workers at the two power plants, with some trapped in a U-shaped 2.7-kilometre  tunnel in Tapovan that filled with mud and rocks when the flood hit.

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.

The semi-liquid debris was estimated to be around 180 metres deep, but rescuers believed there were air pockets in the tunnel, Mr Nair added.

"There was a cloud of dust as the water went by. The ground shook like an earthquake," local resident Om Agarwal told Indian TV.

Yesterday, police had put the number of people missing at more than 200, most of them from the two power plants.

Several hundred rescue workers resumed their search operation at first light today, including national and state disaster response teams, the army and navy diving teams.

Officials said two dams had been emptied to stop the flood waters from reaching the holy towns of Rishikesh and Haridwar where authorities barred people from going near the river.

Villages on hillsides overlooking the river were evacuated, but as night fell authorities said the main flood danger had passed.

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

Prime Minister Narendra Modi said he was monitoring the relief operation. "India stands with Uttarakhand and the nation prays for everyone's safety there," he said on Twitter.

Floods in 2013 in Uttarakhand, which borders Tibet and Nepal, killed 6,000 people, and led to calls for a review of development projects in the state.

Vimlendhu Jha, 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".