BANGKOK - The threat of serious flooding in Bangkok is growing because of the volume of water running down from the centre of Thailand, and one inner city area was under threat on Thursday after floodwater breached a waterworks canal, officials said.

Pracha Promnok, justice minister and head of the government's flood crisis centre, told Channel 3 television that city officials had been pumping water out overnight in the Samsen and Makkasan areas, which are just north of the royal palace and other sites visited by tourists.

"I'm worried about Bangkok residents because if we can't control the situation or things go wrong with the water pumping machine or we can't pump water in time, then there's a chance that our Bangkok will be swamped," Pracha said.

The water level early in the morning was still manageable, he said.

"I have ordered the Metropolitan Waterworks Authority to speed up the production of tap water," he said, adding that it could increase output by about 10 percent per day to 3.9 million cubic metres (138 million cubic feet).

TV showed floodwater flowing into the canal as villagers tried to repair an embankment with improvised sandbags.  

"The contaminated flood water in the canal accounts for only around 5 percent of our raw water and we are now fixing the leak," the waterworks authority said in a statement on its website, reassuring people that tap water was still safe.

Even so, residents of Bangkok rushed to buy bottled water and one central supermarket run by Big C Supercenter Pcl had sold out early on Thursday.

At least 320 people have died in the worst flooding in five decades in the north, northeast and centre of Thailand since late July. About a third of the country's provinces are still affected and some central regions, including the industrialised province of Ayutthaya, are submerged.    

PM under fire

Water from those areas is flowing towards Bangkok and authorities have been desperately trying to divert it around the inner city using a defensive system of dikes and canals.

That worked at the weekend, when the run-off from the north coincided with high tides and heavy monsoon rain, but the risk has risen again.

Residents in seven districts in the northeast of the capital, including some protected until now by a highway that also serves as an embankment, were told to prepare for flooding.

About 200 families were evacuated late on Wednesday and people were told to move cars and valuables to higher ground.  

Pracha said the flow of water down from Nava Nakorn, a big industrial estate north of Bangkok that is completely flooded, and elsewhere in Pathum Thani province was strong but the dikes were holding.

Prime Minister Yingluck Shinawatra, just two months in the job, is facing growing criticism over her government's conflicting messages and slow response to the floods.  

The central bank put the damage from the floods at more than 100 billion baht ($3.3 billion) on Thursday and said it was ready to ease monetary policy if needed.

On Wednesday, its Monetary Policy Committee (MPC) voted to leave the policy rate unchanged at 3.5 percent. It had been raising rates for 15 months to control inflation, which is near the top of its target range.

"The MPC is ready to call a special meeting as it admits that the floods are having quite a severe impact on the real economy," Bank of Thailand Governor Prasarn Trairatvorakul told reporters.  

The cost to the economy could go far higher if Bangkok is swamped. The capital accounts for 41 percent of gross domestic product.  

Already, forecasts for economic growth this year have been slashed. Prasarn said it could be more than one percentage point less than the 4.1 percent the central bank has forecast and Finance Minister Thirachai Phuvanatnaranubala told Reuters on Tuesday growth may be only a bit more than 2 percent.