Rescue teams blasted their way through rockfalls Tuesday as they closed in on the remote epicentre of a Himalayan earthquake that killed 83 people in India, Nepal and Tibet.

More than 5,000 troops, including army engineers using explosives, cleared a route to Mangan, a town near the main impact zone of Sunday's 6.9-magnitude quake on the border between India's northeastern Sikkim state and Nepal.

"The road to Mangan is open," Indian Home Secretary R.K. Singh told reporters in New Delhi.

But officials in the region said it could still take another two days to access isolated villages further north which aerial photos suggested had been badly damaged.

Convoys of vehicles carrying rescue workers, medical teams and emergency supplies left the Sikkim capital Gangtok at daybreak Tuesday.

But progress was tortuously slow over the narrow, badly damaged roads more often frequented by groups of adventurous tourists heading for Himalayan trekking trails.

After covering just a short section of the 60-kilometre (40-mile) route to the worst-affected districts of Mangan and Sangthan, the convoys came to a halt near the town of Phingla, where the path was blocked by a huge rockfall.

As army engineers drilled holes for explosives to blow apart the largest boulders, rescuers waited in frustration along with distraught locals trying to get through to relatives -- unsure whether they were alive or dead.

"I've been here for six hours, waiting for the army to clear the road," said Pema Doma, 37, who has heard nothing from her parents or 16-year-old son in Mangan since the quake hit.

"I'd walk if they would let me," she said. "The anxiety is killing me. What if he's screaming for me? What if he's calling for me and I can't even hear him?"

Those who did attempt walking around the rockfall were stopped by soldiers.

"I know many shortcuts to reach Mangan but the army says it's not safe," said P. Sherpa, 62, whose son is a student at the North Sikkim Academy, a private school in Mangan.

"So all we can do is sit here and stare at the rocks."

Eventually the way was cleared, and the convoys moved slowly on despite the threat of fresh landslides.

A break in the monsoon weather allowed a resumption of helicopter flights which had been grounded most of Monday by heavy rains and low cloud.

Small medical teams with doctors and paramedics were air-lifted into Mangan, and food packages were dropped on some outlying villages.

The death toll from building collapses and landslides in Sikkim stood at 50, but Home Secretary Singh warned the number could rise as emergency workers pressed on towards the epicentre.

Around 300 people have been admitted to hospitals across the state.

The Press Trust of India said 26 tourists, including 15 trekkers, had been rescued and taken to army camps for their own safety.

Around a million people visit Sikkim every year and an estimated 60 percent of the state's tiny population of 500,000 rely on tourism for their livelihood.

"Business will be severely affected," said state tourism official Sam Ten.

Hundreds of Gangtok residents spent a second night out in the open, too frightened to sleep in homes badly damaged by the quake.

Many saw out the night in the city's football stadium, slinging plastic sheets over the goalposts or sleeping on the terraces.

"The stadium is our kitchen and bedroom for the night. We're honestly just too scared to consider anything else," said 32-year-old Amrita Laqandri.

In Nepal, eight people were killed and hundreds of homes destroyed or damaged in the east of the country, where rescuers faced the same problems as their Indian counterparts with rains and mudslides blocking the only highway.

Eighteen 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.

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