Thai military have 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.

Thailand's prime minister has warned people that the country is likely to endure at least one more month of flooding, telling anxious residents in the capital Bangkok to prepare for possible metre-deep water.

The overall flood situation would continue for "four to six weeks", Yingluck Shinawatra, the prime minister, said on Saturday.

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 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.

The authorities have launched a high-stakes attempt to channel the floodwaters from the central plains out to sea through canals in the city, which has already seen waist-high water in parts of its northern outskirts.

Yingluck told Bangkok residents to move their belongings at least one metre off the ground.

The prime minister also called for all agencies to collaborate to protect the capital from a potential deluge of floodwater.

"All agencies have to be united where tackling the runoff is concerned, because successfully diverting the water to drain into the sea via east Bangkok would hinge on all relevant agencies moving in a concerted effort," she said.

Pools of water

Bangkok was clear and sunny for a second day on Saturday, but the threat of heavy rain still loomed large, with defence walls holding for now, and canals already full to the brim carrying millions of cubic metres of water towards the sea.

The floods have killed at least 342 people since July, and devastated seven industrialised areas to the north of Bangkok, inflicting damage estimated at least $3.3bn and putting tens of thousands of Thais temporarily out of work.

Colossal pools of runoff from the north have been bearing down on the capital for the last two weeks.

While Bangkok has mostly survived unscathed, images of disaster just outside the city have unsettled residents, who are girding for the worst.

People in the capital have rushed to stock up on food and bottled water, while motorists have parked hundreds of cars on bridges or elevated roads.

Tens of thousands of soldiers and police have also been mobilised to maintain order.

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