"Due to sudden cloud burst over Lhonak Lake in North Sikkim, a flash flood occurred in the Teesta River... 23 personnel have been reported missing and some vehicles are reported submerged under the slush," the army said in a statement. "Search operations are underway."

The remote area lies close to India's border with Nepal, and Lhonak Lake sits at the base of a glacier in the snowy peaks that surround Kangchenjunga, the world's third-highest mountain.

The army said water released upstream from the Chungthang dam meant the river was already more than 4.5 metres (15 feet) higher than usual.

A video released by an Indian army spokesman showed a thick torrent of raging brown water sweeping down a thickly forested valley, with roads washed away and power lines ripped down.

Flash floods are common during the monsoon season, which begins in June and normally withdraws from the Indian subcontinent by the end of September. By October, the heaviest of the monsoon rains are usually over.

Experts say climate change is increasing their frequency and severity.

- Streets swamped -

Other photographs shared by the army showed water submerging the first floor of buildings, and flowing down a street in a town with only the tip of a small construction crane visible poking out.

Local media showed Sikkim Chief Minister Prem Singh Tamang holding an umbrella during a downpour and talking to officials about floods in the town of Singtam, further downstream from where the soldiers are missing.

The monsoon occurs when summer heat warms the landmass of the subcontinent, causing the air to rise and suck in cooler Indian Ocean winds, which then produce enormous volumes of rain.

But it also brings destruction every year in the form of landslides and floods.

Melting glaciers add to the volume of water while unregulated construction in flood-prone areas exacerbates the damage.

Himalayan glaciers are melting faster than ever before due to climate change, exposing communities to unpredictable and costly disasters.

Glaciers disappeared 65 percent faster from 2011 to 2020 compared with the previous decade, a report in June by the International Centre for Integrated Mountain Development (ICIMOD) warned.

Based on current emissions trajectories, the glaciers could lose up to 80 percent of their current volume by the end of the century, it said.