More than one million people have been trapped by floodwaters in eastern India after heavy monsoon showers caused a major river to shift its course, a minister said Tuesday.

Massive rains in Bihar state caused the Kosi river to swell, breach its banks and flow through a channel it had previously abandoned.

Water levels in the new river, about one mile (1.6 kilometres) across, are not receding, said Bihar Disaster Management Minister Nitish Mishra.

The torrential water washed away villages and small dwellings in its path and trapped "not less than one million people in the widespread flooding," he said.

Four northern districts of the state were the worst hit by the river's changed course.

The UN children's fund UNICEF on Tuesday put the number of people affected by flooding across Bihar, including by earlier monsoon inundations, at 1.4 million in 13 districts of the state.

"The marooned people have to be evacuated to safe places to save their lives after the shift in course in the Kosi river," Mishra told AFP by telephone from Araria, one of the worst-hit districts bordering Nepal.

The force of the water was washing away about 150 metres (yards) of the Kosi's protective levee every day, and the breach was now about two kilometres wide, Mishra said.

Bihar state Chief Minister Nitish Kumar, who visited the inundated areas, called the situation "a catastrophe."

Television footage showed thousands of desperate people trying to negotiate the raging waters by forming human chains as rescue helicopters clattered overhead.

Human bodies were seen floating in the fast-flowing waters, along with animal carcasses and destroyed homes.

The Indian military said it was deploying troops to help with the rescue.

Thousands of victims have been evacuated from Supaul, Araria and Madhepur districts to crowded relief camps, Mishra said, adding hundreds more camps would have to be built.

State authorities sounded an alert as floodwater gushed into Madhepura district's main town, threatening to swamp homes and a crowded prison.

"A batch of army personnel has reached Madhepura for relief and rescue operations along with units of the National Disaster Response Force," a government spokesman said in Patna.

"We have no defences, no sandbags, heavy machinery or even trees to block the gushing waters," said Satendra Kumar, a Madhepura town administrator, as residents fled in panic.

At least 800 people have been killed in monsoon-related accidents during the heavy June-to-September rains across India, with impoverished Bihar accounting for 24 deaths since Saturday, a government spokesman said.

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