Nearly one million people have been evacuated in India's Bihar state where a rain-swollen river burst its banks three weeks ago and swamped hundreds of villages, an official said Monday.

"Some 971,000 people have been evacuated from districts that are flooded," Bihar's disaster management chief Pratayay Amrit said in the state capital Patna.

The official said that around 50,000 more were still marooned in the worst-hit districts of Madhepura and Sapaul, but added that they would soon be brought to safety with the military's help.

"This number will touch one million," he said.

Those plucked from their marooned homes have been housed in schools, temples, student residences and hundreds of temporary shelters, Amrit said.

The Kosi river flowing from Nepal breached its defences on August 18 and shifted away from its normal course, engulfing large swathes of Bihar, India's second most populous state.

Around 100 people are confirmed to have died in the floods, but the real number is certain to be far higher as many were simply washed away by the deep water and strong currents.