PATNA, India, Sept 1 (Reuters) - The Indian army and navy stepped up efforts on Monday to rescue hundreds of thousands of people marooned by floods and facing severe shortages of food in the east of the country.

Some villagers have been living on rooftops for days, while others are eating plants and leaves after exhausting food stocks. Aid agencies said the government of the impoverished state of Bihar should have done more to anticipate the disaster and plan relief operations in a region hit by monsoon flooding every year.

"Lessons from the past disasters should be kept in mind while planning response," ActionAid said in a statement. "A long-term comprehensive response is necessary to deal with relief, recovery and disaster preparedness."

Three million people have been displaced from their homes and at least 90 killed by floods, officials say, after the Kosi river burst a dam in Nepal, swamping hundreds of villages in Bihar and destroying 100,000 ha (250,000 acres) of farmlands.

Hundreds of boats are being used to evacuate people but more are needed, while heavy rains over the past few days have hampered rescue and relief operations, officials said.

"Such is the extent of devastation that the forces deployed are proving too small," Pratyay Amrit, a senior state disaster management official, said on Monday.

The army, which had already deployed five columns of around 120 men, sent in another 14, officials said on Monday, while three naval companies were also asked to help.

The situation on the ground is getting desperate.

"We don't have any more food grain stocks left, and me and my family are all chewing plants and leaves to stay alive," Mohan Sharma said by telephone from Supaul district.

Television pictures showed people fighting to get places in boats, as soldiers in life jackets tried to restore order.

Over 467,000 people have been evacuated so far, but there are thousands still marooned. Activists and local media say the death toll could be many times higher than official estimates.

Some experts have blamed the floods on heavier monsoon rains caused by global warming, while others say authorities have failed to take preventive measures and improve infrastructure. (Writing by Bappa Majumdar; Editing by Simon Denyer) (For the latest Reuters news on India see http://in.reuters.com, for blogs see http://blogs.reuters.com/in/.)

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