About 800,000 people have been displaced and more than 350 have died in the worst flooding in a century in southern India's Kerala state, according to officials.

Key points: 800,000 people are sheltered in some 4,000 relief camps across Kerala

800,000 people are sheltered in some 4,000 relief camps across Kerala Floods have caused landslides and homes and bridges have collapsed

Floods have caused landslides and homes and bridges have collapsed About 10,000 kilometres of roads have been damaged

Downpours that started on August 8 have triggered floods and landslides and caused homes and bridges to collapse across Kerala, a picturesque state known for its quiet tropical backwaters and beautiful beaches.

Authorities are rushing to bring drinking water to the most affected areas.

At least two trains carrying about 1.5 million litres of water were moving to the flooded areas from the neighbouring states of Madhya Pradesh and Maharashtra, Indian railway official Milind Deouskar said, according to the Press Trust of India news agency.

Sorry, this video has expired Indian military rescue mother and baby by helicopter from flood waters (Photo: AP)

And thousands of rescuers were working to reach stranded people and get relief supplies to isolated areas by hundreds of boats and nearly two dozen helicopters, PH Kurian, a top disaster management official in Kerala, said.

He said an estimated 800,000 people were sheltered in some 4,000 relief camps across Kerala.

He said weather conditions had improved considerably and he expected the nearly 10,000 people still stranded to be rescued by Monday.

Sorry, this video has expired Devastating flooding in India has killed hundreds in just nine days

But weather officials have predicted more rains across the state on Monday morning.

Oxfam India spokesman Basab Sarkar said the damage was immeasurable.

"The situation is grim. Thousands are still waiting to be rescued, as rivers are overflowing and houses have been swamped by landslides," he said.

"The magnitude of devastation is immeasurable."

In the Vatican City, Pope Francis expressed solidarity with the people of Kerala, inviting the thousands of faithful gathered in St Peter's Square to join him in prayer for all the victims and people tried by the disaster, asking for concrete support from the international community.

In several villages in the suburbs of Chengannur, one of the worst-affected areas, carcasses of dead cattle were seen floating in muddy waters as water began receding. However, vast rice fields continued to be marooned and many vehicles were submerged.

In some villages, floodwaters up to 3 metres high had entered homes.

Rescuers evacuate people from flooded areas in Aluva. ( Reuters: Sivaram V )

Rescuers in a motorboat reached a hamlet where they tried in vain to persuade an 80-year-old woman, Bhavani Yamma, to be taken to a government-run shelter from her partially submerged single-story house.

"I will not come. This is my home and I will die here," said Ms Yamma, who lives alone.

The team later rescued a 61-year-old kidney ailment patient, Raveendran, who needs dialysis twice a week.

One of the rescuers, Rajagopal, a police constable who uses only one name, said initially "we didn't anticipate it would be such a big disaster". But he said that by Wednesday, "we realised it's really big".

Damage estimated to be worth $3 billion

Officials estimate that more than 10,000 kilometres of roads have been damaged in the natural disaster.

One of the state's major airports, in the city of Kochi, was closed last week due to the flooding. It is scheduled to remain closed until August 26.

The Indian Government said a naval air base in Kochi would be opened for commercial flights starting on Monday morning.

Indian Prime Minister Narendra Modi inspected the floods by helicopter. ( AP: Doordarshan )

Prime Minister Narendra Modi inspected the flooded landscape from a helicopter on Saturday and met with the state's top officials, promising more than $70 million in aid.

While the Central Government has dispatched multiple military units to Kerala, state officials are pleading for additional help.

Officials have put initial storm damage estimates at nearly $3 billion.

More than 1,000 people have died in flooding in seven Indian states since the start of the monsoon season, including more than 350 in Kerala.

Sorry, this video has expired Hundreds killed in India floods as evacuations continue

AP