At least eight people died and 49 others, including 23 army personnel, went missing on Wednesday after a cloudburst over Lhonak Lake in north Sikkim triggered a flash flood in the Teesta River basin, officials said.

Three of the dead were washed up in north Bengal, they said.

The flood in Sikkim that started around 1.30 am was made worse by the release of water from Chungthang dam, the officials said.

“Five bodies have been recovered from Golitar and Singtam region,” Gangtok Sub-Divisional Magistrate (SDM) Mahendra Chettri said.

Another official said besides the 23 army personnel, 26 civilians were also missing, while 45 people rescued, including 18 injured.

“A steel bridge at Singtam, known as Indreni bridge, 30 km from the state capital Gangtok has been completely washed away by Teesta river water early Wednesday,” the official said.

The Sikkim government, in a notification, said the natural calamity has been declared as a disaster.

The release of water from the Chungthang dam led to a sudden increase in water levels up to 15-20 feet high downstream, defence spokesperson Lt Col Mahendra Rawat said.

“Twenty-three Army personnel have been reported missing and 41 vehicles submerged under the slush,” he said.

The spokesperson said the identity of the missing personnel is yet to be ascertained due to a breakdown in communication.

Rajeev Bhattacharya, 25, a tourist from Calcutta who was on a trek from Gangtok towards Singtem, told PTI over phone: “We saw a huge wave of water rushing at great speed through the valley and mangled remains of structures being swept away. Luckily, my friends and I were on high ground and not affected by the flash flood. We are now making our way back to Gangtok.” Several towns, including Dikchu, Singtam and Rangpo located in the Teesta basin, have also been flooded with the upsurge in the river.

Meanwhile, all schools located in Mangan, Gangtok, Pakyong and Namchi districts will remain closed till October 8, the education department said in a circular.

Parts of National Highway-10, the main link between Sikkim and the rest of the country, were washed away, the officials said, adding, a flood alert has been issued for North Bengal and Bangladesh through which the Teesta flows.

“We were travelling to Gangtok from Siliguri when our car was forced to halt in the Swetjhora area. The NH-10 had caved in as incessant rains had eroded the rock and soil below the road. Fortunately, all cars were moving slowly because of the rains, otherwise the cave-in could have actually swallowed a vehicle,” Dolma Bhutia, a college student going back home to Sikkim, told PTI telephonically.

Police officers in neighbouring West Bengal said two youths from Raiganj district -Swarnadwip Majumder (23) and Srikant Majumder (27) and another, Ishan, from Jharkhand - went missing in Sikkim on Wednesday.

The trio had left for the small Himalayan state on Saturday on a motorcycle holiday.

"Since Tuesday morning, they could not be traced as their mobile numbers remained unreachable. We have contacted Sikkim Police for help,” a police officer from Raiganj said.

The officials said two other bridges, Baluatar and another near the Lanco Hydel power project, have collapsed due to the spate in Teesta since Tuesday night.

The administration has set up a number of relief camps, where hundreds are taking shelter, they said.

The Sikkim State Disaster Management Authority stated that a "cloudburst in portions of Lhonak Lake, which caused the rise of water levels with very high velocities downstream along the Teesta River basin in the early hours of Wednesday" damaged many establishments in Mangan, Gangtok, Pakyong, and Namchi.

Chief Minister P S Tamang visited Singtam and took stock of the situation. He also held a meeting with senior officials at the Singtam Nagar Panchayat office and asked them to keep vigil.

Sikkim Chief Minister Prem Singh Tamang takes stock of the situation after flash floods following a cloud burst in Singtam, Sikkim. PTI picture.

In a post on social media, Tamang said his thoughts and prayers are with all during this difficult hour.

“In these challenging times, I want to extend my heartfelt support to all the victims and their families affected by this unfortunate event,” he said.

“I want to assure you that the government is fully committed to providing all necessary assistance and relief to those in need. We understand the magnitude of the situation and are mobilizing all available resources to ensure the safety and well-being of our citizens. Our dedicated teams are working day and night to address the immediate concerns and challenges posed by this calamity,” Tamang said.

In West Bengal, torrential rain in the last few days, besides increase in the water level in the Teesta river, affected several places in Kalimpong, Darjeeling, Alipurduar and Jalpaiguri districts, where the administration has started evacuating affected people to safer locations, state government officials said.