Boats carried emergency supplies to desperate residents of Indonesia's flood-stricken capital yesterday as overflowing rivers again burst their banks following days of rain. At least 20 people have been killed and almost 340,000 others made homeless, officials said.

Hundreds of people remained on the second floors of their houses, either trapped or unwilling to abandon them despite warnings that muddy water running four meters deep in places may rise.

"Jakarta is now on the highest alert level," said Sihar Simanjuntak, an official monitoring the water levels of the many rivers that crisscross the city of 12 million people. "The floods are getting worse."

The government dispatched medical teams on rubber rafts into the worst-hit districts amid fears that disease may spread among residents living in squalid conditions with limited access to clean drinking water.

Edi Darma, an official at Jakarta's Flood Crisis Centre, said the death toll in Jakarta and surrounding towns had reached 20 by last night, mostly by drowning or electrocution. Incessant rain over Jakarta and hills to its south from Thursday triggered the city's worst floods in recent memory. Authorities have cut off electricity and the water supply in many districts.

Racmat Witoelar, the Environment Minister, blamed poor urban planning for the disaster. "Authorities hand out [building permits] even though they clearly violate environmental impact studies," The Jakarta Post newspaper quoted him as saying.

Jakarta's governor Sutiyoso, who was criticised when massive floods struck the city five years ago, blamed widespread deforestation in Puncak, saying it had destroyed water catchment areas.

Dr Rustam Pakaya, from the Health Ministry's crisis centre, said nearly 340,000 people had been made homeless."We fear that diarrhoea and dysentery may break out, as well as illnesses spread by rats," Dr Pakaya said. "People must be careful not to drink dirty water."

There was little rainfall over Jakarta yesterday, but fresh flooding occurred as heavy downpours over the southern hills in Puncak caused rivers to swell across the city, prompting authorities to open flood gates.

Indonesia's meteorological agency is forecasting rain for the next two weeks.