Floods engulfing parts of the Thai capital should start to recede soon, the Prime Minister said on Saturday after barriers along Bangkok's swollen main river prevented a disastrous overflow.

The city of 12 million people was on heightened alert because of a seasonal high tide that was expected to coincide with the arrival of runoff water from the central plains, where people have endured weeks of flood misery.

Prime Minister Yingluck Shinawatra, who has previously warned the floods could last for weeks, said the authorities had sped up the flow of runoff through canals in the east and west of the capital.

"If everyone works hard. Then the floodwater in Bangkok will start to recede in the first week of November," Yingluck said in a weekly radio and television address to the nation.

Yingluck later told reporters she expected the situation "will improve in one or two days".

She added: "Thais must closely monitor the situation during high tide. Please be a bit patient and after that I believe the water level will start to recede because the water flow is easing and part of it is flowing into canals."

Yingluck, the sister of fugitive former premier Thaksin Shinawatra, has been in office for barely two months and her administration has faced criticism for giving confusing advice about the extent of the flood threat.

For a third day running there was minor flooding in Bangkok's riverside areas, including by the Grand Palace, but the high tide of 2.5 metres (eight feet) above sea level was lower than feared and most of the city was dry.

"I'm not too worried. It's only a little bit of water. It's not similar to outside Bangkok," said Sidaphat Ausanarassamee, 32, standing behind a wall of sandbags in front of her shop in the Chinatown area.

"It affects my business. Nobody is buying anything," she added, laughing, as children played in knee-high water in the street and orange-clad monks snapped pictures of the scene with their mobile telephones.

Within Bangkok, residential areas in the northern outskirts of the city, as well as on the western side of the Chao Phraya river have so far been the worst hit, with water waist-deep in places.

The government warned residents in the west of the capital to stockpile tap water because supplies will be limited at times as a result of contamination from rubbish and industrial plants.

The government announced it was moving its emergency flood relief centre from the city's second airport Don Mueang after rising water led to a power blackout.

Tens of thousands of residents have left Bangkok, with many heading to coastal resorts away from the path of the water, after the government declared a special five-day holiday. Yingluck said the break might be extended.

The three-month crisis — triggered by unusually heavy monsoon rains — has left at least 381 people dead and damaged millions of homes and livelihoods, mostly in northern and central Thailand.

The Pentagon said Thailand had asked a US destroyer to extend its stay at a main port to allow two American helicopters to survey the floodwaters.

Most of the country's top tourist destinations and the main airport have been unaffected, although countries including the United States and Britain have advised against all but essential travel to Bangkok.

French tourist Philippe Ponel, 24, on his first trip to Thailand, was among those taking pictures of inundated streets in Chinatown.

"I think the people in Bangkok don't fear the floods at all. They just keep going on with their daily lives. I saw people cooking in the streets with water all around them," he said.