The official death toll from major floods in India's tourist hotspot of Kerala hit 86 on Thursday with media reporting even higher numbers amid warnings of more torrential rainfall.

“At least 21 people have died today. The official toll is now 86,” a Kerala state spokesman told AFP.

With a major rescue operation launched to help thousands of people trapped by the floods, Kerala's chief minister Pinarayi Vijayan said the state now faces an “extremely grave” crisis.

Helicopters airlifted stranded families from rooftops and dam gates were thrown open as incessant torrential rain brought fresh havoc on Thursday to the Indian state of Kerala where about 100 people are feared dead.

Hundreds of extra troops were deployed in the southern state, a major tourist hotspot, as the government issued a “red alert” over the region's worst floods in decades.

At least eight people were reported dead and 15 others, including a three-month-old infant, were trapped inside three houses hit by a landslide near an irrigation dam in Malappuram district, The Hindunewspaper said.

Authorities said many people were trapped inside their houses. More than 60,000 people have sought refuge in relief camps.

“At least 6,500 people are stranded in different parts of Kerala and the situation in three districts is particularly grim,” a Kerala state disaster management official told AFP.

Kerala, famed for its pristine palm-lined beaches and tea plantations, is battered by the monsoon every year but this year's damage has been particularly severe. Floods have also caused havoc in other states, including Karnataka and Madhya Pradesh.

Some 540 army, navy and air forces reinforcements were sent to Kerala on Thursday to join the rescue effort.

The army said it had rescued scores of people with helicopters sent to the region. Defence forces and government boats were also used in an increasingly desperate rescue operation.

Authorities appealed for victims to stand in open fields or on rooftops away from trees so helicopters were not damaged during rescue efforts.

"Please help"

People could be seen paddling lifeboats provided by the military, while in some areas families commandeered local wooden boats to ferry themselves to safety.

Army helicopters rescued families but also dropped food packets and drinking water to some of the worst-affected districts.

The government says 10,000 kilometres of Kerala roads have been destroyed or damaged and hundreds of homes lost.

It has ordered the opening of gates at 34 dams and reservoirs where water levels had reached danger levels.

Cars and livestock washed away in the floods were seen on Indian television, and men and women wading through chest-high waters that had gushed into their homes.

Many used social media to send rooftop distress calls, some with video.

Greeta Mathew pleaded for help for her family in a Twitter message.

“Anybody reading this, PLZ HELP. My relatives are stuck on the upper floor of house with an eight months pregnant lady, in Edayaranmula, Pathanamthitta dist. All rescue control rooms' numbers busy. No rescue team reached yet. No contact with family since last evening,” she said.

North and central Kerala has been worst hit by the floods but all 14 of the state's districts have been put on alert as heavy rain is predicted for several days.

In the main city of Kochi, the international airport was closed until at least Saturday because of flooding. Departures were cancelled while incoming flights have been diverted to other airports in India.

All public transport has been stopped with many buses left abandoned on the road.

Elsewhere, a sudden water surge swept away eight people when it hit a popular picnic spot in Madhya Pradesh state on Wednesday. Another 45 stranded were rescued on Thursday by police.

Prime Minister Narendra Modi said on Thursday on Twitter that he has ordered the defence ministry “to further step up the rescue and relief operations across the state. Praying for the safety and well-being of the people of Kerala”.