Thai troops 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.
Thai prime minister Yingluck Shinawatra has warned it may be six weeks before Thailand's worst floods in decades recede.
She told Bangkok residents to prepare for water up to one metre deep.
Ms Yingluck says the city must open floodgates to allow the water through, the ABC reports.
More than 110,000 evacuees have been forced to seek refuge in 1743 shelters from waters that the government has described as unstoppable.
More than 350 people have been killed in Thailand in three months of heavy rain.
Waters running towards the sea from inundated northern provinces have already swamped some of Bangkok's outer suburbs.