Thailand's prime minister warned Saturday that the kingdom could endure six more weeks of flooding, telling anxious Bangkok residents to be ready for water up to a metre deep in the capital.

A high-stakes effort is under way to drain billions of cubic metres of water from upcountry out to sea through rivers and canals in and around the city, where floods were already waist-high in the northern outskirts.

"Bangkok must open all floodgates to allow the water through," said Prime Minister Yingluck Shinawatra, who has invoked a disaster law to take full control of the emergency response.

"So during this long weekend residents should move belongings, cars and other valuable things to places at least one metre (three feet) high," she said in her televised weekly address to the nation.

"It's an extremely serious situation that affects people's lives and property," added Yingluck, whose two-month-old government is racing to avert a humanitarian disaster.

More than 1,000 people have been evacuated from their homes in the northern Bangkok districts of Don Mueang and Lak Si, where the water was up to 70 centimetres deep, making roads impassable for small cars.

There was also minor flooding near the parliament district after the water level of the Chao Phraya River rose, prompting the city authorities to advise 3,000 people living alongside its banks to consider moving to shelters.

"Water in the river is extremely high," said Bangkok Metropolitan Administration spokesman Jate Sopitpongsthorn.

Three months of heavy monsoon rains have killed at least 356 people in Thailand and damaged the homes and livelihoods of nine million people, mostly in the north and centre, the government said.

About 113,000 people have been forced to seek refuge in shelters, Yingluck said, adding that while the waters were receding in some areas of the country, the floods heading towards the capital were unstoppable.

"There is a huge volume of run-off water from the north and we can't effectively block it but can only slow the flow because our barriers are temporary," she warned.

The overall flood situation would continue for "four to six weeks", she added.

The centre of Bangkok was still dry on Saturday, and sunny blue skies belied the sense of nervous dread in the city of 12 million people.

Supermarkets were 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.

Tens of thousands of soldiers and police have been mobilised to maintain order, but the military warned people who refuse to leave their inundated homes not to rely on troops to bring them essential items.

"People must help themselves too. It's not practical if they just wait for relief workers who number in the hundreds of thousands, while there are millions of victims," army chief General Prayut Chan-O-Cha told reporters.

The military airlifted 14 critically ill patients from Bangkok's Don Mueang airport 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 Organisation representative to Thailand and the acting United Nations Resident Coordinator.

"Bangkok faces the same needs as for other flooded areas but in a far higher quantity," she said, noting the 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."

The opposition is calling on the government to declare a state of emergency to make it easier to control people, but Yingluck has ruled out such a move.

She said the authorities would secure important locations such as the palaces, government buildings, major utilities and key transport routes.

A political novice before taking office, the sister of fugitive former leader Thaksin Shinawatra is facing the first major test of her fledgling leadership.

Her administration has been accused of initially responding slowly to the crisis and of giving contradictory information.

The disaster has dealt a heavy blow to Thailand's export-dependent economy, with hundreds of factories outside the capital flooded.

Thailand's top tourist destinations as well as the capital's main airport have so far been unaffected.