Rescue teams battled mudslides and torrential rains Monday to reach victims of a strong earthquake that rocked a remote Himalayan region, killing 63 people in India, Nepal and Tibet.

The epicentre of Sunday's 6.9-magnitude earthquake was an isolated area of the border between India's Sikkim state and Nepal, and there were fears the toll could rise as reports filtered in from distant towns and villages.

The heavy rains and low cloud grounded helicopters, and Indian relief and rescue teams trying to access the Sikkim state capital Gangtok had to clear dozens of landslides blocking the only viable highway.

"The biggest challenge now is to get the rescue teams to the affected areas," said Sikkim Information Minister C.B. Karki.

Thousands of troops were deployed to help clear the way to Gangtok and beyond, and by early evening trucks carrying rescue and relief supplies began to get through to the city after a long, perilous drive.

Stretches of the route were lined by locals and townspeople who had evacuated their homes and were too scared to return because of multiple aftershocks and rockfalls.

"We just had a landslide right here," said village shopkeeper Karma Mantor. "We are still in a state of panic. Nobody knows what is really happening."

The death toll in Sikkim stood at 35, with five people killed in Gangtok and the others dying in building collapses and rockfalls in outlying areas, including two soldiers on road-clearing duty.

"We cannot rule out more casualties," Indian Home Secretary R.K. Singh told a news briefing in New Delhi.

"There might still be villages where people are trapped. Sikkim is a hilly area and villages are far-flung," Singh said.

Tremors were felt more than 1,000 kilometres (600 miles) away in New Delhi to the west, and in Bangladesh to the east.

In Nepal, police said a motorcyclist and his eight-year-old daughter were among three killed when a perimeter wall crumbled at the British embassy compound in the capital Kathmandu, 270 kilometres west of the epicentre.

Five others were killed in separate incidents in eastern Nepal.

More than 100 people were injured by mudslides, falling debris and collapsing buildings in Gangtok, where thousands faced another cold night in the open -- too frightened to return to their damaged homes.

"People are still very worried and tense. Everything is shut down and nearly everyone is still out in the street because they're scared of another quake," said Gangtok resident Indira Singh.

There was some relief as power, cut off by the quake, was restored to the city by Monday afternoon, but land line and mobile communications remained erratic, especially in the worst-affected areas.

Although the highway to Gangtok was largely cleared, roads heading north to Mangan near the epicentre were still impassable, posing a further challenge to rescuers and relief workers.

"I have family up there and I'm worried," said Namiya Tsering, 24. "We haven't been able to establish any contact with them."

The quake was felt across a wide region including the Himalayan kingdom of Bhutan after it struck at about 6:10 pm (1240 GMT) on Sunday, according to the US Geological Survey.

Its epicentre was just over 60 kilometres northwest of Gangtok, at a relatively shallow depth of 19.7 kilometres.

The Press Trust of India said police rescued 15 foreign tourists in the north of Sikkim, a popular destination for trekkers.

Thirteen other people died in the Indian states of Bihar and West Bengal, while China's official Xinhua news agency said seven people had been killed in southern Tibet, near the border with Sikkim.

Nepalese police spokesman Binod Singh said hundreds of homes were damaged in eastern Nepal, where rescue workers faced the same problems as their Indian counterparts with rains and mudslides blocking the only highway to the area.

India's seven northeastern states, joined to the rest of the country by a narrow sliver of land known as the "chicken's neck," are located in an area of frequent seismic activity.

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