Bangkok: The death toll from Thailand's worst flooding in decades reached 506 on Sunday, as flood waters threatened to inundate more areas in downtown Bangkok, the Interior Ministry said.

The Thai Disaster Relief and Mitigation Department said the flooding has wreaked havoc in 25 provinces including Bangkok and nearby provinces. Authorities ordered evacuations for residents in eight of Bangkok's 50 districts by Sunday.

Floodwaters were approaching a main road near Bangkok city's Mo Chit bus terminal, a major gateway to northern Thailand, traffic police chief Uthaiwan Kaewsa-ard said.

Across the country, the flooding -- now in its third month -- has affected 25 of Thailand's 64 provinces.

Bangkok's central business district is still dry but other parts of the bustling metropolis of 12 million people are inundated.

In Bangkok, floodwater on the Vibhavadi-Rangsit road just a few kms from downtown was 70-80 centimeters high in the afternoon.

At least 20 percent of the mega city is already submerged in floodwater raising fears about outbreaks of water borne diseases in the densely populated metropolis.

Efforts are on to steer the floodwaters through canals in the east and west of Bangkok out to the river.

The floodwaters have already inundated the busy Lat Phrao intersection on the fringe of the city centre while the hugely popular weekend market Chatuchak, which has been hit by flood waters, remained open on Sunday though many traders stayed away.

The floods are a result of unusually heavy monsoon rains that began three months ago.

(Agencies)