"Due to sudden cloud burst over Lhonak Lake in North Sikkim, a flash flood occurred in the Teesta River... 23 personnel have been reported missing and some vehicles are reported submerged under the slush," the army said in a statement.

"Search operations are underway."

The remote area lies close to India's border with Nepal, and Lhonak Lake lies at the base of a glacier in the snowy peaks that surround Kangchenjunga, the world's third highest mountain.

The army said water released upstream from the Chungthang dam meant the river was already more than 4.5 metres higher (15 feet) higher than usual.

Flash floods are common during the monsoon season, which begins in June and normally withdraws from the Indian subcontinent by the end of September. By October the heaviest of the monsoon rains are usually over

Experts say climate change is increasing their frequency and severity.