At least 79 people have been killed and 2.2 million forced to leave their homes by flooding caused by torrential monsoon rain across India's north east.
Assam state, which borders Bhutan and Bangladesh, has been worst hit, with the massive Brahmaputra river breaching its banks.
Extensive flooding has also hit the states of Arunachal Pradesh, Manipur and Meghalaya.
The Assam state government said 26 of 27 districts had endured flash floods as heavy rains destroyed thousands of flimsy homes, blocked roads and swamped fields.
"The people of Assam are facing one of the worst floods in recent times that has inflicted considerable damage," Prime Minister Manmohan Singh said in the state capital Guwahati after touring the area by helicopter.
"The central and state governments are doing everything possible to provide relief to the people," he added.
The Assam government said in a statement that an estimated 2.2 million people had been displaced, with thousands of homes wrecked and more than 500,000 people being sheltered in relief camps.
"So far 79 people have died in separate incidents of boat capsize or have drowned while trying to escape the gushing waters and also in landslides," it said.
Assam's health minister Himanta Biswa Sarma said: "We have opened makeshift relief camps for the displaced, while many more were forced to take shelter on raised platforms and in tarpaulin tents."
Officials said more than 70% of the Kaziranga National Park, famous for its tigers, one-horned rhinos and elephants, was submerged.
"The animals are trying to move to safer areas," said park warden Sanjib Bora.
In the adjoining states of Arunachal Pradesh, Manipur and Meghalaya, monsoon rains caused widespread flooding but there were no reported deaths.
The death toll from landslides and flooding in Bangladesh over the last week has risen from 108 to 123, officials said, though floodwaters were declining.
Abdul Wajed, head of Bangladesh's National Disaster Response Co-ordination Centre, said: "Around 900,000 people have been affected by floods in five districts of the country on both sides of the Brahmaputra. We have mobilised relief for the affected people."
The monsoon, which sweeps across the sub-continent from June to September, is crucial for the region's farmers but also claims casualties from flooding every year.