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Bangkok, Thailand (CNN) -- Bangkok faces the highest flood levels yet and is preparing for the worst, the governor of the Thai capital told CNN.
Residents are urged to flee the rising floodwaters, which have already forced the closure of Bangkok's Don Muang airport and the evacuation of flood victims who have taken refuge there.
Thailand's government has declared a five-day public holiday in flood-affected provinces to try to encourage people to seek safety elsewhere before high tides expected this weekend.
But Bangkok Gov. Sukhumbhand Paripatra told CNN the authorities could not evacuate a whole city and it was difficult to persuade the Thai people to leave their homes, despite the risk. CNNGo: Updated info for tourists
"Apparently there will be large volume of water runoff coming toward the city tonight onwards, and over the weekend," he said. "At the point of high tide, it will be very high, the highest this year. We are bracing for the worse."
Thongthong Chantharangsu, a spokesman for Thailand's Flood Relief Operations Center, appealed on TV for Bangkok residents to head to the countryside.
Floodwaters extend from Rangsit, north of Bangkok, to Don Muang airport and Yingcharoen Market, state-run news agency MCOT reported.
The water has reached 30cm (12 inches) in places and is overflowing on to sidewalks and some roads, causing problems for small vehicles and leading to traffic congestion, the agency said.
In a televised address Tuesday night, Thai Prime Minister Yingluck Shinawatra said the capital could be submerged by as much as 1.5 meters (4.9 feet) of water.
Of particular concern were areas along the Chao Phraya River, which winds through the capital and is expected to overwhelm embankments this weekend.
The Airports of Thailand declared Don Muang airport, which primarily services domestic flights, closed Tuesday night, after floodwaters flowed onto runways and affected the lighting.
Nok Air, which usually operates from Don Muang, was forced to cancel flights but should be able to run an almost normal schedule by Friday after moving its operations to the main Suvarnabhumi Airport, the airline's chief executive Patee Sarasin said Wednesday. Some 3,000 Nok Air passengers were affected by flight cancellations Tuesday, he said.
The flood relief operation will continue to be based at the airport, the Thai government said Wednesday.
More than 600 prisoners held at Bang Kwang Central Prison have been evacuated, according to the Department of Corrections. The high-security prison has about 4,000 inmates, the chief of the prison said, some of them high-profile.
The floods have also forced the Dusit Zoo to evacuate some animals, including goat antelope and Sika deer, to a zoo in the countryside, according to Dusit Zoo's chief, Karnchai Saenwong.
The U.S. ambassador to Thailand, Kristie A. Kenny, said the crisis was slow moving and it was hard to know what would be hit next.
The United States has already provided civilian relief resources including water pumps, purifiers and life vests, she said, and two U.S. helicopters are helping the Thai military survey the extent of the flooding.
Nationwide, the floods have killed 373 people and affected more than 9.5 million people, authorities said.
The public holiday announced Tuesday will be from Thursday to next Monday and will be effective in 21 provinces, including Bangkok, that are still under water, a government spokeswoman said.
The government has called the flooding the worst to afflict the nation in half a century and said it might take more than a month before the waters recede in some areas.
The government has set up more than 1,700 shelters nationwide, and more than 113,000 people have taken refuge.
Overall damage from the floods has risen and could top $6 billion, with the worst yet to come as the waters destroy shops and paralyze factories nationwide, the Thai Finance Ministry said.
Thailand derives a significant portion of its revenue from tourism, which has been diminished by the flooding.
CNN's Elizabeth Yuan, Kocha Olarn and Paula Hancocks contributed to this report