A 6.8-magnitude earthquake struck the Sikkim-Nepal border area late Sunday evening and sent shockwaves across north and east India and Bangladesh.
At least six persons were killed in the northeastern state - though the toll could be much higher, sources said - and 50 injured.
In Nepal , five persons were killed and 44 injured.
The West Bengal districts of Darjeeling and Jalpaiguri were also badly affected with one death reported in the hill town.
The quake struck an area in Sikkim 64 km from the capital , Gangtok, at 6.10pm and was followed by three strong aftershocks measuring 5.7, 5.3 and 4.6 on the Richter scale.
The tremors were felt far and wide - from Assam to Delhi, Meghalaya to Bengal - and snapped power and communication lines across the region.
The Met department predicted more aftershocks but said "the magnitude and frequency will reduce".
As the disaster struck after sundown and with phone lines down, there was no accurate information on the extent of damage to life and property.
The army and air force was deployed in Sikkim for rescue and relief operations but their work was hampered by the rain. Met department officials it was likely to continue to rain for the next 24 hours.
Prime Minister Manmohan Singh spoke to chief minister Pawan Kumar Chamling and offered all help.
He also directed the cabinet secretary to call an emergency meeting of the National Disaster Management Authority, PMO sources said.
In Darjeeling, a power substation collapsed, plunging the hills into darkness. Till late in the night, people refused to go back to their homes.
"I am worried about Darjeeling," Bengal chief minister Mamata Banerjee said.
"The possibility of a major quake in north Bengal is quite high and earthquakes can cause landslides," said Met office director GC Debnath.
In Kathmandu, three persons were killed when a boundary wall of the British embassy collapsed.
This is the strongest quake to have rocked the country in 77 years. In 1934, an 8.4-magnitude quake had killed 7,000.