An estimated 200 people are missing and 18 are confirmed killed after an avalanche in India's Himalayan region broke dams, swept away bridges, and left dozens of construction workers trapped in tunnels.

Here are answers to some questions on the disaster:

The incident began on Sunday morning below Nanda Devi, India's second-highest peak.

India's power minister, R.K. Singh, said an avalanche led to flash floods that swept away the small Rishiganga hydro electric project and damaged a bigger one further down the Dhauliganga river being built by state firm NTPC.

Video footage showed a torrent of water, rock and dust sweeping down a mountain valley, where workers were still constructing and maintaining the dams.

WHAT ARE THE POSSIBLE CAUSES?

While it is too early to conclusively determine how the disaster began, experts said heavy snowfall followed by bright sunshine led to excessive snow-melt, triggering a chain reaction that led to the avalanche.

"The area witnessed a heavy snowfall and then solar rays resulted in the melting of ice," said Ravi Chopra, director of the non-profit People's Science Institute in Uttarakhand state, where the incident took place.

"On Sunday morning it was a bright, sunny day, and some of the snow started melting, which possibly led to an avalanche," said Himanshu Thakkar, coordinator of the South Asia Network of Dams, Rivers and People, a research group.

Vikram Gupta, a scientist at the federal government's Wadia Institute of Himalayan Geology, said investigators were studying whether a rise in temperatures led to the avalanche, but results would not be known until later in the week.

HAS THIS HAPPENED BEFORE?

Incidents like Sunday's are rare so early in the year, but avalanches and flash flooding in the Himalayas are common during summer and monsoon months, as snow melt and heavy rains combine.

In June 2013, record monsoon rains in Uttarakhand caused devastating floods that claimed close to 6,000 lives.

A government report said while the disaster occurred as a result of natural hazards, human activity had contributed significantly.

"The disaster revealed several infirmities in our preparedness, which need to be rectified at the earliest," the report said, calling for limiting construction within flood plains and restrictions on blasting.

WERE THESE RECOMMENDATIONS FOLLOWED?

In 2019, residents of Raini - one of the villages worst affected by Sunday's avalanche - approached Uttarakhand's top court, local media reported, asking the state government to investigate what they said were consistent breaches of the guidelines in the area.

The court ordered local officials to investigate the claims.

But experts said little has been done to curb the use of dynamite in construction, which significantly weakens the surrounding hills.

"It would definitely have had a major impact," Thakkar said. "There have been clear violations."

Om Prakash, Uttarakhand's top government bureaucrat, declined to comment on claims guidelines had been breached.