Thai authorities urged residents in flood-prone areas of Bangkok to evacuate Wednesday, warning them that the arrival of a massive deluge of water was imminent.

A huge runoff from the north equivalent to 480,000 Olympic swimming pools is expected to reach the capital at the same time as seasonal high tides, the UN's Office for the Coordination of Humanitarian Affairs said.

"The floodwater has reached the inner city," it said in a statement, putting the expected volume of water at 1.2 billion cubic metres.

The government has ordered a five-day holiday from Thursday for 21 provinces including Bangkok, to allow the city's residents to prepare for the inundation or leave.

"I would like to ask Bangkok people who are already affected or could be affected soon to consider evacuating to other places," said Thongthong Chantarangsu, spokesman for the country's Flood Relief Operations Centre (FROC).

"Food and deliveries will become more difficult," he added. FROC said it was working on providing extra shelters across the city of 12 million people.

Bangkok governor Sukhumbhand Paribatra said that a large amount of water "will get to Bangkok tonight", after Prime Minister Yingluck Shinawatra said flood barriers might not be able to hold back the deluge.

During the holiday Government offices will be closed and authorities have urged public and private companies to allow their staff the time off, but the central bank said financial markets and banks would not shut.

Public schools in Bangkok, currently on holiday, are not expected to reopen until November 15, and several international schools have also delayed the return of pupils.

More than 600 inmates, including some on death row, were moved on Wednesday from the notorious Bang Kwang prison on the Chao Phraya riverbank -- better known as the "Bangkok Hilton".

Workers at Dusit Zoo, also near the waterway, also began to evacuate some of its residents, including spotted deer and antelopes, while structures were provided for lions to climb up to safety.

Adding to the deluged kingdom's woes are fears of crocodiles on the loose from flooded farms -- another three were captured Wednesday in Nonthaburi province, north of the capital.

"They are between 1.7 and 1.8 metres (five and a half and six feet) long," an official from the fisheries department told AFP, adding that they knew of no attacks on humans by the escaped reptiles, whose numbers remain unclear.

The country's worst flooding crisis in decades has been plagued by contradictory information from national and local authorities, confusing Bangkok residents who have been bracing for the onslaught of water for days.

Yingluck said on Wednesday she was "fifty percent confident that the inner zone of Bangkok will not be completely flooded," toning down comments from a day before when she said flooding in central Bangkok was "highly likely".

She said a "worst-case scenario" would see parts of the low-lying city inundated by "more than one metre" of water.

"Initially, the floods in Bangkok will last for two weeks to one month," she added.

Many supermarkets were running low on essential items such as bottled water and eggs as residents stocked up on goods ahead of the expected deluge, and the premier advised people to boil tap water before consuming it.

More than 370 people have been killed in the three-month flood crisis caused by unusually heavy monsoon rains, damaging the homes of millions of people and forcing tens of thousands into evacuation centres.

Bangkok's main international airport, Suvarnabhumi, is operating as normal but the domestic Don Mueang airport in the north of the city suspended flights on Tuesday until November 1, after waters started seeping onto the runways.

About 4,000 flood evacuees who had found refuge at a disused terminal on the compound were being moved to other shelters.

The disaster is expected to shave around one percent off Thailand's economic growth in 2011, according to the government, which on Tuesday announced a raft of measures to help flood-affected businesses in an bid to reassure investors.

Proposals to boost investor confidence include about $10 billion in soft loans and tax waivers.