More than 300 people have been killed in the Indian state of Kerala by the worst flooding to hit the region in a century, authorities said Friday, as troop reinforcements stepped up rescue efforts.

The state -- a key international tourist draw with its tropical mountains and beaches -- has been battered by record monsoon rainfall in recent days and is "facing the worst flood in 100 years", chief minister Pinarayi Vijayan said.

In a Twitter statement released by his office, he said there had been "324 lives lost", a tripling of the death toll in the last 24 hours.

Vijayan added that 223,139 people have been moved to more than 1,500 relief camps.

With thousands still trapped, power and communication lines down and fresh alerts of further torrential rain, authorities warned of even more trouble ahead.

More than 30 military helicopters and 320 boats are attempting rescues across Kerala.

Extra troops have been sent in and local fishermen have also joined the operation with their boats.

People all over the state of 33 million people have made panic-stricken appeals on social media for help, saying they cannot get through to rescue services.

India's Prime Minister Narendra Modi was set to arrive in the stricken state on Friday night.