Thai Prime Minister Yingluck Shinawatra said Thursday that it was impossible to protect all of Bangkok from the country's worst floods in decades, describing the situation as a "national crisis," Bangkok Post online reported.

A total of 320 people were dead in the floods that have inundated the upper part of the country for almost three months. The flood has affected about nine million people, mostly in the north and central provinces, while authorities are struggling to keep the capital safe.

"We cannot block the water forever," Bangkok Post quoted Yingluck as saying, adding that the government would choose which parts of the city to allow the water through to minimise the impact.

"The longer we block the water the higher it gets," she said. " We need areas that water can be drained through so the water can flow out to the sea."

Inner Bangkok has so far escaped major flooding as the authorities divert water to areas outside the main capital in a bid to prevent the Chao Phraya River from bursting its banks and flooding the political and economic heartland.

But efforts to keep the city of 12 million people dry have been complicated by a seasonal high tide. Moreover, there have been cracks on several in Bangkok, according to Royal Irrigation Department official.

Earlier on Wednesday, Bangkok Governor issued evacuation warning for Bankokians living outside flood barrier in the north of Bangkok to evacuate. Also, people living in seven districts in the north and east of Bangkok were warned to prepare for flooding.