Tens of thousands of people have fled to safer areas but more than a million were still trapped in floods after a river changed its course due to heavy rains, an official said Thursday.

At least two million people were marooned when the Kosi river breached its banks and flooded huge swathes of the eastern state of Bihar.

"About 90,000 victims have been evacuated from villages in the flood affected area by government rescue agencies," disaster management official Prataya Amrit told AFP.

More than 400 boats had been pressed into service and hundreds more would be used to shift people to relief shelters and higher land, he said.

At least 46 people are reported to have died in the floods.

Army troops and air force helicopters were helping police in the rescue operation.

Thousands of residents abandoned their homes after the river started shifting course on August 18 and have taken shelter in crowded relief camps or in high-rise buildings.

On Wednesday, the government said one million tonnes of rice and wheat would be distributed to stranded people.

Prime Minister Manmohan Singh, Home Minister Shivraj Patil and the chief of ruling Congress party Sonia Gandhi arrived in Bihar on Thursday to survey the region and review the aid effort.

Monsoon downpours caused the Kosi to swell, burst through its flood levees on the Nepal border and flow through a channel it had previously abandoned.

Nepalese disaster management officials told AFP the river had washed away a series of dams and spurs, which control the water, sending huge torrents downstream that washed way further flood defences.

Authorities on both sides of the border have been in dispute over maintenance of flood control structures and uncleared silt, officials said.

More than 800 people have been killed in monsoon-related accidents following the heavy June-to-September rains across India.

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