Thai troops on Sunday reinforced vulnerable barriers along Bangkok's Chao Phraya river after a sudden rise in the water level set back efforts to contain the worst floods in decades.

Prime Minister Yingluck Shinawatra has warned it could take six weeks for the deadly flooding to recede, telling residents in the capital to prepare for possible inundation up to one metre (three feet) deep.

Three months of heavy monsoon rains have killed more than 350 people in Thailand and damaged the homes and livelihoods of nine million people.

More than 110,000 evacuees have been forced to seek refuge in 1,743 shelters from waters that the government has described as unstoppable.

The authorities are battling the slowly advancing floods on several fronts, with parts of the northern outskirts of Bangkok already under waist-deep water.

Now the level of the Chao Phraya river is causing "great concern", the government said.

The river briefly overflowed Saturday, causing flooding in parts of five central Bangkok districts -- including near the political heartland -- prompting a race to fix the leaks before the next high tide on Sunday evening.

The Bangkok authorities have urged about 3,000 people living on the banks of the Chao Phraya on the river side of the flood barriers to move to emergency shelters.

In the north of the city, meanwhile, the floods were closing in Bangkok's second-largest airport, Don Mueang, part of which has been turned into a refuge for thousands of evacuees and is defended by walls of sandbags.

More than 150 schools in the capital have also been prepared as emergency shelters.

The government is desperately trying to drain billions of cubic metres of water from upcountry out to sea through rivers and canals in and around the city by opening all sluice gates.

"There are several factors that we can't control. The water is coming in two directions," Yingluck told reporters.

"Bangkok residents will be affected because water will flow down, particularly through the Saen Saeb canal (which crosses the city centre) and the drainage system which will help water to be diverted out quickly."

Another major test is expected between October 28 and 30 when seasonal high tides flow up the Chao Phraya, meeting run-off water from the north.

More than 1,000 people have been evacuated from their homes in the northern Bangkok districts of Don Mueang and Lak Si because of overflow from canals in the area.

"But the situation seems to be under control as we see less water overflowing and the military is working to stop the leaks," Don Mueang district chief Phumpat Damrongkiatisak told AFP.

Supermarkets in the city have been busy with people stocking up on food and bottled water, while many motorists have moved their cars to the upper levels of multi-storey carparks or left them on bridges and elevated roads.

Most of Thailand's top tourist destinations and the capital's main airport have so far been unaffected.

Tens of thousands of soldiers and police have been mobilised to maintain order and the military has airlifted about a dozen critically ill patients from Bangkok to the eastern province of Chonburi as a precaution.

Aid organisations were also gearing up to assist the city's residents, said Maureen Birmingham, World Health Organization representative to Thailand.

UN agencies "are aware that there may be a big need for survival commodities," she said.

The government's flood relief centre said that floodwater in Pathum Thani province, just north of Bangkok, had been found to contain toxic chemicals and pollution experts were investigating the cause.

Conditions are desperate for flood victims who stayed behind, some using home made floating devices in waters up to their necks.

"I've never seen anything like this in my life," Boonchu Thongtha told AFP in Rangsit, Pathum Thani where the water is chest deep.

"I can't go to the toilet, I can't do anything. My biggest fear is to not have food to eat. Our stored food is finished. I don't know what to do. We have nine people in the family," the 62-year-old told AFP.