Rescue teams have closed in on the remote epicenter of Sunday's earthquake that killed at least 81 people in northeastern India, Nepal and the Chinese region of Tibet, but many communities remain cut off.

Multiple landslides, fog and heavy rain had prevented many rescue workers from reaching the impact zone. On Tuesday, air force helicopters were able to fly rescue and medical teams into one of the worst affected districts in the Indian state of Sikkim. The sparsely-populated mountainous state felt the full impact of the magnitude 6.9 quake that struck two days ago.

More than 5,000 army troops also blasted though rockfalls to clear a major highway leading to Mangan, a town near the epicenter.

Still, authorities say it could be days before they have access to remote villages in northern Sikkim. India's Home Secretary R.K. Singh said Tuesday that helicopters were being used to survey the damage and warned the death toll could increase.

Most of the Indian deaths from the quake occurred in Sikkim and the remainder were killed in Bihar and West Bengal states. Many of the victims were killed by the collapse of buildings that had already been weakened by heavy monsoon rains.

Nepalese authorities reported eight quake-related deaths, while China's official Xinhua news agency said at least seven people were killed in southern Tibet.