Thailand's hundreds of thousands of flood victims are at risk of water-borne diseases and infections, the World Health Organisation said Saturday, though no major outbreaks have been reported yet.

The spread of communicable diseases such as diarrhoea, respiratory illness and conjunctivitis among displaced flood victims in shelters was a key concern, the country's WHO representative Maureen Birmingham told AFP.

Flood-affected people also faced an increased risk of skin fungal infections and leptospirosis, a bacterial infection spread through contaminated water, she said.

"Those risks are there," said Birmingham, who is also the acting United Nations Resident Coordinator in Thailand. "But thus far there have been no reported major outbreaks."

The biggest danger facing the flood victims was drowning, she said, adding that residents should take care when fishing and they should be aware of the risks of electrocution and snake bites.

Three months of unusually heavy monsoon rains have inundated large swathes of the country, killing more than 350 people and forcing tens of thousands of families to seek refuge in evacuation centres.

Bangkok, a city of 12 million people, has so far escaped the brunt of the nation's worst flooding in decades, but run-off water from the north has started seeping into parts of the city and locals braced for worse to come.

"Bangkok faces the same needs as for other flooded areas but in a far higher quantity," said Birmingham, noting that the densely populated capital was highly dependant on supply chains that might be interrupted.

"A lot of (UN) agencies are aware that there may be a big need for survival commodities."

Birmingham advised residents to "have a good stockpile of meds and a supply of hygiene materials", adding that this was especially important for people with chronic conditions.