Heavy rain and thick cloud hamper relief operations after more than 100,000 homes in India, Nepal and Tibet were hit

Rescuers are battling heavy rain and thick cloud as they try to reach villages cut off by mudslides after a powerful earthquake killed at least 63 people and damaged more than 100,000 homes in India, Nepal and Tibet.

Thousands of soldiers and rescue workers continued to pull victims from rubble as the number of deaths climbed to 35 in the north-eastern Indian state of Sikkim, site of the epicentre of the 6.9-magnitude quake that rattled through the Himalayas on Sunday night.

Thick cloud prevented helicopters from flying over parts of the affected areas, but some mountain passes blocked by landslides had been reopened, police said.

"The earthquake has loosened the hill faces, and when it rains it causes landslides. So the situation is still very dangerous," said Deepak Pandey, a spokesman for the Indo-Tibetan border police. More than 400 people had been rescued overnight, he said, including some in the worst-hit area of Pegong in the north.

Although it may take days for the final number of fatalities to be confirmed, border police said they did not think the death toll would rise significantly.

More than 6,000 troops have been drafted in to clear concrete slabs, bricks and mud and reach scores of people trapped under collapsed houses.

India's home secretary, Raj Kumar Singh, said that airforce helicopters had dropped food to villages, airlifted a medical team, evacuated the injured and conducted damage assessments. Heavy construction equipment had also been used to clear some of the blocked roads.

"The rescue and relief operations are in full swing, though they were hampered … by poor weather," he said, "[but] there may still be villages where people are trapped under collapsed houses that we have not been able to reach."

Singh said that at least 10 of those who died in Sikkim worked for the same hydroelectric project. At least 13 other people were killed in the neighbouring Indian states of Bihar and West Bengal, he added. Eight people died in Nepal, and China's official Xinhua news agency reported seven deaths in Tibet.

Most of the deaths occurred when houses already weakened from recent monsoon rains collapsed because of the force of the quake.

By midday on Monday, workers had managed to clear landslides from one lane of the main highway connecting Sikkim with the rest of India, and an initial convoy of 75 paramilitaries had started moving toward Mangan, the village closest to the quake's epicentre, officials said.

In Gangtok, Sikkim's capital, 42 miles south-east of the epicentre, police said they had cordoned off the office of the state's top elected official after the building was severely damaged. TV footage showed buckled buildings, cracked pavements and two major roads collapsed. Shops, businesses and offices were closed in the town and neighbouring settlements, and many areas remained without electricity. Water supplies were scarce because of burst pipes and telephone communication was patchy.

Nepal's government said at least eight people died there, including two men and a child who were killed when part of the perimeter wall of the British embassy compound in Kathmandu collapsed. A spokesman for the UK Foreign Office confirmed the incident, adding that it "deeply regrets" the death of the three Nepali citizens and the injuries to others.

"The ambassador met with and offered his condolences to the local community on Sunday evening and met with relatives of the victims on Monday[yesterday] morning," he said. "The embassy will continue to do everything possible to help the local community and the government of Nepal."

In West Bengal, utility workers toiled through the night to restore power to a large swath of the state which plunged into darkness after power lines were snapped.

The earthquake, which was followed by several aftershocks, was felt as far away as the Indian capital. An official from the UN's disaster management team in New Delhi said that humanitarian assistance would still be needed even if casualties turned out to be low, as people who lost their homes would need food and shelter.

The region has been hit by major earthquakes in the past, including in 1950 and 1897.