Flash floods and landslides triggered by heavy monsoon rains in northeast India have claimed at least 26 lives, with eight other people missing.

The army and air force has launched rescue operations in areas badly hit by the rains that have been lashing the states of Sikkim, Assam and Arunachal Pradesh for days.

In a remote area of northeastern state of Sikkim, at least 21 bodies, including those of members of the Indo-Tibetan Border Police, the Press Trust of India news agency said on Sunday.

In Assam state, five deaths were reported in widespread flooding that submerged a major wildlife sanctuary and parks, the news agency said.

Karma Zyatso, chief secretary of Sikkim state, said the workers, who lived in camps, were swept away by the swirling waters.

He also said helicopters dropped food supplies to nearly 10,000 people living in six villages cut off due to damage to the main highway in the area, about 550km west of Gauhati, the capital of Assam.

The rains have swollen the mighty Brahmaputra river and its tributaries, causing floods that have affected hundreds of thousands of people in the region.

The air force was dropping food supplies to people marooned by floods.

India's northeast has been especially hard hit by devastating rains during the monsoon, which runs from June to September.

Locals expressed grief on losing their property due to the landslide.

"My house has been swept due to the landslide and everything I owned is ruined," lamented a local, Kipa Bhutia.