* Indian army and navy steps up rescue work in Bihar
* Villagers stuck on rooftops, eating plants and leaves
* Rivers break embankments in Bangladesh
* Dhaka warns of worse to come as major river levels rise
PATNA, India, Sept 1 The Indian army and navy stepped up efforts on Monday to rescue hundreds of thousands of people marooned by floods, while rising river levels also rang alarm bells in neighbouring Bangladesh.
In India's impoverished eastern state of Bihar, villagers have been living on rooftops for days, while others are eating plants and leaves after exhausting food stocks.
Aid agencies said the Bihar government 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 in Bihar, 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.
Since the monsoon began in South Asia in June, more than 1,000 people have died in floods, with most of the casualties recorded in India's northern state of Uttar Pradesh in July.
In Bihar, 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.
In India's northeastern state of Assam, rising rivers broke mud embankments and swamped more than 100 villages, forcing 50,000 people from their homes.
Two people drowned overnight, raising the death toll in the northeast region to 34.
Authorities said at least 400,000 people were living in knee-deep water in villages along the border with Bangladesh.
In Bangladesh, about half a million people were marooned after several rivers in the north and central districts broke their banks and swamped villages and farmlands.
The Flood Forecasting Centre warned the flood situation could worsen in the next couple of days as the country's three major rivers were rising alarmingly.
Many schools were closed and turned into relief camps. (Additional reporting by Biswajyoti Das in Guwahati and Ruma Paul in Dhaka; Writing by Bappa Majumdar; Editing by Simon Denyer) (For the latest Reuters news on India see in.reuters.com, for blogs see blogs.reuters.com/in/.)