The government said the disaster has now killed 506 people nationwide (AFP/File, Pornchai Kittiwongsakul)

BANGKOK — The death toll from Thailand’s worst floods in decades jumped above 500 on Sunday as the seemingly unstoppable waters crept deeper into Bangkok, swamping main roads and threatening the city centre.

The government said the disaster has now killed 506 people nationwide — an increase of 60 from the figure reported a day earlier. So far no deaths in Bangkok have been reported in the official toll.

At least 20 percent of the megacity is already submerged in floodwater contaminated by rubbish, dead animals and industrial waste, raising fears about outbreaks of disease in the densely populated metropolis of 12 million people.

With the slowly advancing water now just a few kilometres (miles) away from Bangkok’s business and tourists districts, authorities are seeking to push the floods through waterways in the east and west of the capital and out to sea.

Prime Minister Yingluck Shinawatra said Saturday that the city’s economic and political heartland would stay mostly dry and was only at risk of “minor and brief” flooding.

She said a six-kilometre (four-mile) flood wall of huge sandbags had been erected in northern Bangkok and more water pumps had been installed in other parts of the city to keep the downtown area dry.

The floodwaters have already inundated the busy Lat Phrao intersection on the northern edge of the city centre, forcing the closure of the Central Plaza shopping mall.

Nearby Chatuchak weekend market — a popular tourist attraction — was open for business on Sunday but many traders stayed away after warnings from officials to be on alert for possible inundations.

Hundreds of thousands of residents have been told to evacuate a number of Bangkok districts but many have chosen to stay despite risks including electrocution, disease and lack of food and drinking water.

The floods, triggered by unusually heavy monsoon rains that began three months ago, have damaged the homes and livelihoods of millions across the kingdom.