New Delhi - More than 50 people were killed and an estimated 1.1 million people were marooned in India's eastern state of Bihar after the Kosi river flooded and changed its course following heavy monsoon rains, officials said Thursday.

"Over fifty people have died in the worst-hit districts of Supaul, Madhepura, Araria and Saharsa since Tuesday evening as the death-toll during the monsoons in the state nears 100," said Anil Kumar, an official in the state's disaster management department.

Indian Prime Minister Manmohan Singh who took an aerial survey of the districts where the swollen Kosi river has wreaked havoc called the floods as a "national calamity."

He announced immediate assistance of 10 billion rupees (234 million dollars) and 125,000 tonnes of food grain for the affected population.

According to state officials, the river's high waters caused multiple breaches and flooded new areas, marooning 1.1 million people in the four districts. Many of those have been without food and water for several days.

"Thousands of people and children are struggling to survive. They are on the top of roofs without any food or potable water," a witness told the CNN-IBN news channel.

"Government helicopters and boats are just not reaching them in time to evacuate the people," he said.

Road links, water and electricity supplies to the affected districts have been cut off and rail operations have been suspended as tracks are submerged under water.

About 2.6 million people in 15 districts are affected by the impact of one of the worst floods in the state in recent memory. All major rivers in the state, including Ganga, Ghagra, Burhi and Gandhak, are flowing above the danger mark.

More than 1,644 people died in this year's monsoon rains across India, in the states of Assam, Arunachal Pradesh, West Bengal, Orissa, Jharkhand and Uttar Pradesh, the federal Home Ministry said.

India's monsoon season lasts from June to October but came a few weeks early this year. (dpa)