A sudden cloud burst over Lhonak Lake in North Sikkim resulting in a flash flood in the Teesta River in Lachen Valley on Wednesday, which was compounded by the release of water from a dam, leading to 23 army personnel being washed away, camps and vehicles being submerged, officials said.

The floods started around 1.30 AM, they said.

Defence officials said establishments along the valley have been affected and efforts are on to confirm details.

The release of water from the Chungthang dam led to a sudden increase in water levels up to 15-20 feet high downstream, they said.

This has led to army vehicles parked at Bardang near Singtam getting affected, Defence spokesperson Lt. Col Mahendra Rawat said.

Twenty-three army personnel have been reported missing and 41 vehicles submerged under the slush, he said, adding that search operations are underway.

Border Road Organisation (BRO) has already launched rescue operations and 80 locals have been safely evacuated so far, the defence officials said.

A steel bridge at Singtam, also known as Indreni bridge, in Gangtok district has been completely washed away by Teesta river water early on Wednesday.

The 120-metre-cable suspension bridge is a very important thoroughfare over the Teesta river.

"All the affected people of Singtam town have been taken to safe places and residents of Dikchu village along the river belt have been evacuated to a nearby school," a Sikkim government official said.

Chief Minister PS Tamang visited Singtam to assess the damage caused by the flash flood in the Teesta river basin, he said.

The CM held a meeting with senior officials at the Singtam Nagar Panchayat office and asked them to keep vigil, he said.

The government is actively working to address the challenges posed by the flooding and provide necessary support to those affected by the natural calamity, the official said.

The water level increased drastically overnight, affecting the Dam site of the 510 MV NHPC hydroelectric power project near Dikchu village, he added.