A search operation is currently going on.
The sudden cloud burst reportedly occurred over Lhonak Lake.
Some army establishments, which are located along the valley, have been affected and efforts are on to confirm details.
"Release of water from the Chungthang dam led to the sudden increase in water level upto 15-20 feet high downstream," a source said.
Twenty-three personnel have been reported missing and some vehicles are reported submerged under the slush.