The death toll from heavy monsoon rains which have caused massive flooding in India's northeast has risen to more than 120, with six million forced to flee their homes, officials said Saturday.

The weather office forecast that more rains during the next 24 hours would lash the region, which is suffering from its worst flooding in recent years.

Assam state has been hardest hit by the annual rains with the mighty Brahmaputra river overflowing its banks, while flooding has also struck the nearby states of Arunachal Pradesh, Manipur and Meghalaya.

"So far a total of 121 people have died in separate incidents in which 105 were drowned while trying to escape the gushing waters and 16 more were killed in landslides caused by heavy rains," an Assam government statement said.

An estimated six million people have been forced to leave their homes to escape the floodwaters and find higher ground, a separate Central Water Commission bulletin said.

The monsoon, which sweeps across the subcontinent from June to September, is crucial for India's farmers but also claims many casualties from flooding every year.

Assam state officials were struggling to cope with the huge number of people displaced by the flooding, with makeshift relief camps sheltering some of those forced to leave their homes.

Twenty-six of the 27 districts in the tea-and-oil-rich state have been hit by flash floods since June 24 as a result of the torrential rains while the Brahmaputra river has breached its banks in at least nine places.

The flooding has also devastated the Kaziranga National Park, famous for its tigers, one-horned rhinos and elephants.

The Press Trust of India quoted officials as saying more than 540 of the park's animals, including 13 rhinos, had died.

In the adjoining states of Arunachal Pradesh, Manipur and Meghalaya, monsoon rains have caused widespread flooding but there have been no reported deaths.

Indian Prime Minister Manmohan Singh toured the Assam region by helicopter earlier in the week to view the effects of the rains.

"The people of Assam are facing one of the worst floods in recent times that has inflicted considerable damage," Singh said afterwards.

While India's northeast has received too much rain, the monsoon has been late arriving in other parts of the country.

The monsoon is dubbed the "economic lifeline" of India, which has a population of 1.2 billion and is one of the world's leading producers of rice, sugar, wheat and cotton.

Millions of Indian farmers still rely on monsoon rains to water around 60 percent of the country's farmland.

This year, the monsoon rainfall is running at 31 percent below the normal annual average. But the weather office has forecast heavy rains in the key planting months of July and August to make up for the shortfall.