Heavy rain hampered the search for 23 Indian troops reported missing on Wednesday after a flash flood in northeastern Sikkim state, with the capital city of Gangtok cut off by road, defence officials said.

“Twenty three personnel have been reported missing and some vehicles are reported submerged under the slush. Search operations are underway,” a defence ministry spokesperson said.

India’s weather department has warned of landslides and disruption to flights as heavy rain is predicted in some parts of Sikkim.

The flooding was caused by intense rain triggering a glacial lake outburst, experts at the International Centre for Integrated Mountain Development, an intergovernmental scientific authority based in Nepal, said.

“We observe that such extreme events increase in frequency as the climate continues to warm and takes us into unknown territory,” Miriam Jackson, a senior cryosphere specialist at ICIMOD said.

The cloudburst over Lhonak lake was the main trigger for the flooding in the valley located about 150 km (93 miles) north of Gangtok near the border with China.

The main highway linking Sikkim to West Bengal state was snapped and Gangtok was totally cut off by road, according to army spokesperson Anjan Basumatary.

About 15,000 people living in the vicinity are likely to be affected and at least eight major bridges have been washed away along the banks of the Teesta river, V S Pathak, chief secretary of the state, told Reuters.

Media showed images of the river lashing through the hills and entering human habitations.

Some army bases and facilities along the valley were affected and rising water submerged vehicles following the release of water from a dam, the defence ministry spokesperson based in the neighbouring city of Guwahati said.

Intermittent rain and thunderstorms were hampering rescue operations in the area, an army official told Reuters, speaking on condition of anonymity.