Frustrated flood victims berated Thailand's under-pressure prime minister on Thursday during a visit to inundated areas of the capital Bangkok, one-fifth of which is now under water.

The authorities have advised more than one million people to evacuate but many have chosen to stay despite risks including electrocution, disease and lack of food and drinking water, complicating relief efforts.

Prime Minister Yingluck Shinawatra, facing the first major test of her fledgling leadership, visited flood victims in hard-hit Don Mueang district in northern Bangkok where she was rebuked by disgruntled residents.

"I don't know if you've come here to help or make the situation worse," shouted a woman who missed out on an aid package because supplies ran out.

During a boat tour of areas submerged by polluted floodwaters an elderly man told Yingluck: "You're here just for fun, not really to help, so don't come back!"

The sister of ousted premier Thaksin Shinawatra has been under intense pressure since taking office less than three months ago and has shown signs of emotional strain.

"I have a heavy heart seeing people suffer, while I have to coordinate with many people in my work, so it's stressful," she told reporters at the government's flood relief centre later in the day.

"Anyway I will work to the best of my ability and please be assured that all officials will comply," she added.

While the centre of the capital remains dry, some northern and western parts have been submerged in dirty water that is waist-deep or higher in places.

"In terms of area about 20 percent of the capital is under flood water but nobody knows the exact population affected," said a spokesman for the Bangkok Metropolitan Administration, Jate Sopitpongstorn.

"There are 11,000 evacuees living in temporary shelters across the city."

Nationwide, 437 people have been killed in the disaster, though so far there have been no official reports of deaths in Bangkok.

The authorities have issued an evacuation order for eight Bangkok districts out of a total of 50 in the capital, and for certain areas in four others.

The 12 districts have a combined official population of 1.7 million people -- far more than government shelters can accommodate.

Worst-hit residents have complained that their homes are being sacrificed to save downtown Bangkok's shopping malls, luxury hotels and the houses of the wealthy elite, triggering protests and the destruction of some dykes.

The authorities are attempting to drain the floods through waterways in the east and west of the sprawling metropolis, which is home to 12 million people.

Officials have vowed to do their utmost to protect the centre of Bangkok from inundation, but have been criticised for giving confusing information about the threat level for inner parts of the capital.

The floods -- caused by three months of unusually heavy rains and failure to release enough water from dams in the early part of the monsoon -- have damaged the homes and livelihoods of millions of people around the country.