The dead include four from the ITBP, two of their family members, and 12 junior officers from the BRO. Other victims were casual labourers, officials said.
Most of the casualties are reported from Rangma range.
Landslides were caused by heavy rains since September 19, Deputy Commissioner North district T W Khangsherpa said.
There were about nine landslides blocking the main road between Chungthang and Mangan in the district, he said.
Army and IAF personnel have started rescue operations in full swing.
Several landslides blocked the main roads between Chungthang and Pegong. Houses along Lachen river between Rangma and Chungthang were washed away in flash floods along Lachen river.
The state government has declared a state of alert along the establishments on the river banks right from Lachen, Lachung, Chungthang, Mangan, Dikchu, Singtam and Rangpo, a government press release said.
In the small hours of today, another landslide was reported from a place 1 km away from Mangan, the district headquarter town of North Sikkim.
As a result of which, official reports said NH31-A has been blocked leaving the northern side of the Himalayan state cut off.
