Kerala, famed for its pristine palm-lined beaches and tea plantations, is battered by the monsoon every year but the rains have been particularly severe this season, flooding hundreds of villages and prompting the authorities to suspend flights in and out of the region.
"Twenty five deaths were confirmed earlier in the evening. The situation is bad in many parts of the state and the number (of deaths) will likely increase," a senior official from the Kerala State Disaster Management Authority told AFP.
Unconfirmed local media reports mentioned a much higher toll and cited ongoing efforts to reach many flooded areas.
The official added that authorities had issued a "red-alert" warning in all 14 districts of the flood-ravaged state, with heavy downpours expected over the next 24 hours in many of the worst affected areas.
Kerala Chief Minister Pinarayi Vijayan's office announced on Twitter that Kochi International Airport -- the main gateway to the region -- would be closed until Saturday "due to heavy rains and resultant flooding".
Vijayan held emergency meetings with rescue officials in the state late Wednesday and discussed the situation with Prime Minister Narendra Modi.
The chief minister has requested the deployment of additional personnel from the Indian army, navy and other emergency forces who are already working across the state, which is home to around 33 million people.
"Our state is in the midst of an unprecedented flood havoc," Vijayan said earlier this week. "The calamity has caused immeasurable misery and devastation."
