The death toll from flash floods and landslides in and around Indonesia's capital Jakarta reached at least 21, with more heavy rain forecast, authorities said.

The deadliest floods in years displaced more than 30,000 people and caused chaos across parts of Southeast Asia's biggest city with train lines blocked and power outages in some areas.

Social affairs ministry spokesman Joko Hariyanto said in a message to Reuters that the death toll had now reached 21.

Swathes of Jakarta and nearby towns were inundated after heavy rain fell on Dec. 31 and into the early hours of New Year's Day.

Indonesia's geophysics agency forecast rain accompanied by lightning and strong winds later on Thursday.

Authorities did not give a full breakdown of the causes of death for all of the victims, but earlier said four people had drowned, four died in landslides and four more were electrocuted, while three died of hypothermia.

Jakarta and its surroundings are home to more than 30 million people. More than 50 people died in one of the capital’s deadliest floods in 2007 and five years ago much of the centre of the city was inundated after canals overflowed.

City authorities have in the last few years sought to improve low-lying Jakarta’s vulnerability to flooding during the rainy season.