Saharsa: Massive flooding in eastern India has caused a "national calamity", prime minister Manmohan Singh said on Thursday, after touring the devastated region where more than a million remain trapped.

The premier surveyed the four most-seriously-hit districts - Supaul, Saharsa, Araria and Madhepura, in a helicopter with Congress president Sonia Gandhi.

Singh announced a relief package of 228 million dollars and 125,000 tonnes of grain for those affected when a monsoon-swollen river changed course, flooding huge swathes of the country's impoverished Bihar state.

"If there is a need for more, we will give more," he told reporters. "We would like to assure the people of Bihar that all India will support them through this difficulty."

At least 55 people are reported to have died and some two million displaced after the Kosi river breached its banks, changing course.

Army troops and air force helicopters rushed to help police in the rescue operation.

Thousands of residents abandoned their homes as the floodwater spread and many have taken shelter in crowded relief camps or in buildings on higher ground.

Bihar officials said the death toll could climb further as many areas were inaccessible.