The death toll from worsening monsoon floods in India's Assam state has hit 18, with hundreds of thousands in makeshift camps and no letup in the deluge, officials said.

"Monsoons are still ongoing. There are fresh developments every hour," Rajib Prakash Barua, a senior Assam State Disaster Management Authority official, told AFP news agency on Thursday.

The 18 people have died since the monsoons started in the northeastern state in April, said Barua, speaking from the state's main city, Guwahati.

"One person died yesterday, taking the overall death toll in the last week to five. Most people died either because of flooding or electrocution," he added.

Officials say about 395,000 people have so far been affected in 863 villages across Assam's 15 districts.

Many people have moved to makeshift camps set up by authorities on higher ground.

Jorhat and Sonitpur, around 300km and 190km from Guwahati, are among the worst affected districts in the state. More storms were predicted for Friday and coming days.

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

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

Also on Tuesday, an Indian air force helicopter flying food supplies to remote areas went missing in the state in bad weather. Its wreckage was found a day later near the country's border with China.