Videos broadcast on Indian media showed a terrifying surge of muddy water sweeping away multi-storey apartment blocks in the tourist region of Dharali in Uttarakhand state.

Several people could be seen running before being engulfed by the dark waves of debris that uprooted entire buildings.

Uttarakhand State Chief Minister Pushkar Singh Dhami said rescue teams had been deployed "on a war footing".

Senior local official Prashant Arya said four people had been killed, with other officials warning that the number could rise.

India's army said 150 troops had reached the town, helping rescue around 20 people who had survived the wall of freezing sludge.

"A massive mudslide struck Dharali... triggering a sudden flow of debris and water through the settlement," the army said.

Images released by the army, taken from the site after the main torrent had passed, showed a river of slow-moving mud.

A wide swathe of the town was swamped by deep debris. In places, the mud lapped at the rooftops of houses.

"Search and rescue efforts are ongoing, with all available resources being deployed to locate and evacuate any remaining stranded persons," army spokesman Suneel Bartwal said.

Prime Minister Narendra Modi expressed his condolences in a statement, and said that "no stone is being left unturned in providing assistance".

Chief Minister Dhami said the flood was caused by a sudden and intense "cloudburst", calling the destruction "extremely sad and distressing".

The India Meteorological Department issued a red alert warning for the area, saying it had recorded "extremely heavy" rainfall of around 21 centimetres (eight inches) in isolated parts of Uttarakhand.

Deadly floods and landslides are common during the monsoon season from June to September, but experts say climate change, coupled with urbanisation, is increasing their frequency and severity.

The UN's World Meteorological Organization said last year that increasingly intense floods and droughts are a "distress signal" of what is to come as climate change makes the planet's water cycle ever more unpredictable.