Prime Minister Yingluck described the crisis as "extremely serious," reports Agence France Presse.
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During a weekly televised address, she warned that flooding in the capital was now inevitable:
"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 [...] Bangkok must open all floodgates to allow the water through."
Some northern parts of the city are already flooded, reports the BBC's correspondent in Bangkok. Authorities are seeking to channel floodwaters through canals to the sea to protect the city center, reporter Rachel Harvey says, but there may be simply too much water for the plan to work.
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Residents must brace themselves for waters up to a meter high, Yingluck said, urging them to move cars and possessions to higher ground.
The warning contradicts her assurance last week that Bangkok was not at risk of floods, notes Reuters, which says that the new prime minister has been criticised for sending mixed signals to the public in its handling of the crisis.
The floods have killed at least 356 people and displaced around 113,000 people so far, according to the government's estimate.
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