Heavy rains since the start of India's monsoon season have triggered floods and landslides in parts of the remote northeastern region, causing at least 20 deaths, authorities said Wednesday.

The deaths included drowning or fatal injuries from collapsed homes in Assam state, where the mighty Brahmaputra River was flowing above the danger level, swelled by heavy rain in the past two weeks, said the state's disaster management authority.

Nearly 400,000 people have left their flooded homes in 750 villages across nearly half of Assam's 27 districts. Nearly 30,000 people have taken shelter in relief camps run by the state government. Most others were living with their relatives or on nearby river embankments or higher ground.

On Tuesday, the authority said it evacuated 400 students after their school campus was flooded in western Nalbari district, 90 kilometres west of Gauhati, the state capital.

In neighbouring Arunachal Pradesh state, roads to several districts have been cut off due to incessant rain and landslides.

An Indian air force helicopter flying food supplies to remote areas went missing in the state in bad weather Tuesday, said India's junior Home Minister Kiren Rijiju.

Rijiju himself had a narrow escape when his helicopter was caught in a storm and heavy rains and force-landed in Itanagar, the capital of Arunachal Pradesh.

The monsoon season runs from June through September, and deadly floods occur regularly. Drowning, electrocution or injuries from collapsed houses killed at least 175 people across India during the season last year.