Flash flood hit North Sikkim district and submerged the National Highway 10 at Pegong, officials said on Thursday.
Heavy rains lashed North Sikkim on Thursday night which resulted in the overflow of nearby rivers leading to the flash flood that engulfed the highway.
Areas such as Lachen and Lachung in North Sikkim have been cut off from the rest of the State due to flash flood, they said.
The flash flood has caused severe damage to infrastructure along the highway, making it unsafe for any kind of vehicular movement, officials said.
Local authorities are currently assessing the situation and Border Roads Organisation is working to clear the debris and restore normalcy.
However, it is expected to take some time before the highway can be reopened for traffic, they said.
The road is also blocked due to landslide at 13th mile and Thulo Khola, Rail khola along JN Road of Gangtok-Nathula route.
All tourist permits have been cancelled for Tsomgo Lake, Baba Mandir, Nathula and North Sikkim till further notice, they added.
