DIG Vineet Vinayak said 15 members of the Border Road Organization (BRO), five Indo-Tibet Border Police (ITBP) personnel and two civilians working in the Teesta-Urja hydel project were killed in the flash flood. The Pawan Chamling government has sounded alert along river banks in Lachen, Lachung, Dikchu, Singtam and Rangpo.
Two major bridges on the stretch were also destroyed, severing the sole link between Gangtok and tourist hotspots of Lachen, Lachung and Yumthang.
The disaster comes a year after a 6.9-magnitude earthquake hit Sikkim last September that killed 70 people and destroying the villages. Its epicentre was located at Chungthang.
Officials said it's unlikely tourists would be trapped in those towns as permits are not issued for travel to these areas during the rainy season.
"Till now, 30 persons are reported missing, but the numbers could increase as we have not been able to access the area between Mangan and Chungthang yet," said Vinayak.
BRO Sikkim chief engineer S S Porwal and his team are stranded in north Sikkim without food, equipment or electricity. "The entire stretch of highway between Singhik and Chungthang has been washed away. We have lost all heavy road-building machinery. Other teams have to look for alternative routes and build roads and bridges afresh," Porwal said.
The Army has been called, but officials said incessant rainfall was slowing down relief and rescue operations. Four choppers could not take off in heavy rain.