Indian emergency services on Wednesday rushed supplies to 2.5 million people marooned after heavy monsoon rains caused a river to shift its course, a minister said Wednesday.

Junior agriculture minister Akhilesh Prasad Singh said one million tonnes of rice and wheat would be doled out after the Kosi river breached its banks and flooded huge swathes of the eastern state of Bihar.

The military said it had deployed helicopters and soldiers to reach stranded people after monsoon downpours caused the Kosi to swell, burst its flood levees and flow through a channel it had previously abandoned.

"A couple of days ago just a handful of districts were affected and today there are 15 which are under water," Mukesh Kumar told an AFP correspondent in Madhepura, some 225 kilometres (140 miles) from the state capital Patna.

"We need a massive rescue effort," said local council member Kumar.

Bihar state authorities on Wednesday put the number of people affected at 2.5 million, with at least 46 dead.

Among those hit was Shanti Devi, who dragged her two children out of their house as the waters rose.

"The government is asking us to escape but we have nowhere to go," the 32-year-old mother said.

Some 10,000 of Madhepura's 45,000 residents fled soon after Kosi began shifting from its course on August 18.

Bihar Chief Minister Nitish Kumar, in an overnight address on state-run radio, simply advised panic-stricken people to "run."

"Time is running out for these luckless people," warned state Revenue Minister Narendra Narayan Yadav in Madehpura, where the swirling water has overrun train tracks and the main highway.

People were using rowboats or a few backroads still above the rising water levels to flee.

The force of the water was washing away about 150 metres (yards) of the Kosi's protective levee every day, Bihar Disaster Management Minister Nitish Mishra warned.