At least 20 people died when a double-decker tourist boat capsized in India's southern state of Kerala late Sunday, authorities told local media.

The incident happened in Tanur, a coastal town in Kerala's Malappuram district. Rescue operations were ongoing.

The Times of India newspaper and the PTI news agency put the death toll at 20, while the Indian Express daily listed it at 18.

The state's sports and fisheries minister V Abdurahiman, who was helping to coordinate rescue efforts, said most of the victims were children on school holidays. More than 30 people were believed to be on the vessel at the time of the incident.

Abdurahiman said four people were taken to hospital in a critical condition, according to PTI.

Survivors told local media that many of the passengers were not wearing life jackets.

"Pained by the loss of lives due to the boat mishap in Malappuram, Kerala. Condolences to the bereaved families," Prime Minister Narendra Modi said on Twitter, adding that the next of kin of each victim would be entitled to compensation.

It was not immediately clear why the boat capsized.