New Delhi  - Indian authorities, aided by the military, intensified their efforts Monday to rescue more than a half-million people marooned by floods in the eastern state of Bihar.

Medical teams were rushed in to prevent outbreaks of diseases in the affected areas.

An estimated 3.5 million people were affected or displaced when the monsoon-driven Kosi River changed its course after almost two centuries after a breach in a dam upstream in Nepal.

Nearly 80 people were killed in Bihar's 16 districts. Saharsa, Supaul, Araria, Purnea and Madhepura districts were among the hardest hit.

Authorities said 1.1 million people were marooned as two weeks of flooding continued in 1,600 villages in the region.

"Rescue and relief operations are going on full-steam with all three wings of the defence forces joining efforts of the state administration," Prataya Amrit, Bihar's top disaster management official, said by telephone from state capital, Patna.

"We have evacuated 550,000 people from the affected areas till now," he said. "In the next two to three days, we will be able to rescue 200,000 flood victims, requiring maximum attention in critical areas."

Amrit added that tens of thousands of flood victims had moved to safer places on their own.

More than 250,000 people were lodged in more than 200 relief camps run by government and volunteer agencies. Others took shelter with family or friends, he said.

Nearly 6,500 military personnel were involved in the relief and evacuation efforts, and 500 army engineers were clearing and widening water channels to ease the Kosi's flow.

Meanwhile, a dozen helicopters were air-dropping food. About 750 large and inflatable rubber boats were pressed into service for the evacuation efforts, the NDTV network reported.

About 1,500 medical personnel and doctors fanned out to the affected areas as well as overcrowded and unsanitary relief camps, where outbreaks of water-borne and infectious diseases were feared.

"The situation in various relief camps is precarious because food supplies are not reaching [them]," an aid-worker, Sandeep Jain, said. "Disease could break out as several hundreds are cramped in small spaces."

But the situation was expected to improve as the river flow from Nepal eased.

"Hundreds of villages are still under water, but the scenario looks better as Nepalese and Indian authorities are in constant touch to tame the river," Amrit said.

Prime Minister Manmohan Singh, who visited the affected areas last week, called the floods a "national calamity" and announced an aid package of 234 million dollars.

In Bihar's neighbouring state of Uttar Pradesh, 814 people have been killed in monsoon-driven floods this year.

In all, 1,736 people have died in this year's monsoon rains across India, federal Home Affairs Ministry officials said. India's monsoon season lasts from June to October but came early this year. (dpa)