The flood, caused by heavy rainfall, blocked the main highway to Sikkim’s capital, Gangtok.
Heavy rain left cars and an army base covered in mud and slush. The Defense Ministry said search operations are under way.
The downpours caused the banks of a glacial lake to burst, creating a weather disaster, said Miriam Jackson, a research scientist at the International Centre for Integrated Mountain Development in Nepal.
“We observe that such extreme events increase in frequency as the climate continues to warm and takes us into unknown territory,” she said.
Local officials also say a dam overflowed, exacerbating the flooding.
Around 15,000 people will likely be impacted and at least eight major bridges were swept away, according to India’s Secretary of State V.S. Pathak.
The sudden rain and thunderstorms have delayed rescue operations, an army official told Reuters on condition of anonymity.
Some information for this report was provided by Reuters.