More than 324 people have died in the worst flooding in nearly a century in the south Indian state of Kerala.

Roads are damaged, mobile phone networks are down, an international airport has been closed and more than 220,000 people have been left homeless after unusually heavy rain in the past nine days.

Officials have repeatedly revised the death toll upwards from 86 people on Friday morning to more than 300 by the evening as a massive rescue operation reached more flood-hit regions. “Around 100 people died in the last 36 hours alone,” a state official said.

Casualty numbers are expected to increase further, with thousands more people still stranded and less intense though still heavy rain forecast for at least the next 24 hours. Many have died from being buried in hundreds of landslides set off by the flooding.

The Kerala chief minister, Pinarayi Vijayan, said the state was experiencing an “extremely grave” crisis, with the highest flood warning in place in 12 of its 14 regions.

“We’re witnessing something that has never happened before in the history of Kerala,” he told reporters.

Kerala, famed for its tea plantations, beaches and tranquil backwaters, is frequently saturated during the annual monsoon. But this year’s deluge has swamped at least 20,000 homes and forced people into more than 1,500 relief camps.