Authorities are struggling to rescue tens of thousands of people who remain stranded in flood-ravaged eastern India.

Officials Saturday said a boat searching for survivors in the state of Bihar capsized - pushing the official death toll from the flooding to more than 80.

Aid workers say the death toll is likely much higher and is expected to rise, as flood victims struggle without food, adequate, shelter and clean water.

More than two million people have been impacted since the rain-swollen Kosi River breached its embankments last week.

The United Nations Children's Fund says the flooding has disrupted life in more than 1,000 villages in northern Bihar, and has destroyed about 250,000 homes.  UNICEF officials say it could be months before people can return to the area, because floodwaters have severely damaged roads, and water and electricity supplies.

India's government has established camps for displaced people, but a UNICEF spokeswoman, Miranda Eeles, says conditions at the sites are poor. 

On Thursday, Indian Prime Minister Manmohan Singh called the flooding a "national calamity," and he pledged millions of dollars for emergency relief efforts.

In a statement issued Saturday, United Nations Secretary-General Ban Ki-moon sent his condolences to the families of flood victims.

Officials say the flooding also has displaced at least 70,000 people in Nepal, where the Kosi begins.  Nepal and India have pledged to work together on relief efforts.

South Asia's monsoon season runs from June to September and is vital to the region's agriculture.  But heavy rains also cause hundreds of deaths and widespread destruction every year.

At least 800 people have been killed so far this year.

Some information for this report was provided by AFP, AP and Reuters.