The worst monsoon floods in a decade to hit a remote northeastern Indian state have killed more than 80 people and forced around 2 million to leave their homes, officials said Monday.

Nearly half a million people are living in relief camps that have been set up across Assam state, Prime Minister Manmohan Singh told journalists in Gauhati, Assam's capital. The rest of the 2 million displaced have moved in with relatives or are living in the open, sheltering under tarpaulin sheets.

Assam officials say 81 people have been killed over the past four days. Most of them were swept away when the mighty Brahmaputra River overflowed its banks and flooded villages. Sixteen people were buried in landslides triggered by the rains.

At least 11 people were missing in six districts, the state disaster management agency said in its bulletin. Air force helicopters were dropping food packets and drinking water to marooned people, Singh said after surveying the flood-hit districts.

Army soldiers used boats to rescue villagers from rooftops of flooded homes. Teams of doctors have opened health clinics in the 770 relief camps that had been set up across Assam, one of India's main tea-growing states. The hilly tea growing areas have not been affected, but lower rice fields have been washed away.

Thousands of cattle have perished after being swept away by the raging water or getting stuck in the mud. The stench of rotting animal carcasses was adding to the woes of the people in tents at the relief camps, officials said. In the worst-hit Dhemaji district, raging waters of the Brahmaputra River swept away entire villages.

Officials said the entire Majuli island, one of the world's largest river islands, was awash as water levels in the Brahmaputra rose above the danger level.

"This is one of the worst floods to hit Assam," Singh said. He announced the national government would give immediate assistance of 5 billion rupees ($90 million) to the state.

Railway workers were working round the clock to restore train services disrupted after railway tracks became submerged in flood water.

"Restoration of the railway line is a priority," Singh said. Officials say the situation was expected to improve over the next few days as the rain was tapering off and water levels were beginning to recede.

Monsoon floods hit Assam, with a population of 26 million people, almost every year, with heavy rains swelling the Brahmaputra and its innumerable tributaries that crisscross the state.