BANGKOK— Heavy downpours that caused rivers to burst around Thailand have killed 57 people in nearly two weeks of flooding that officials are calling the worst in decades, authorities said Wednesday.

The flooding has affected more than 3 million people in 36 of Thailand’s 76 provinces, government medical and disaster agencies said. The floods have eased in a third of those provinces.

More than 4 million sandbags were used to erect walls this week in Bangkok along stretches of the Chao Phraya River, which has swelled with runoffs from upper provinces that officials feared could inundate the capital.

So far, flooding in Bangkok has been minimal but riverside residents were warned to be on alert through Friday, after which current high-tide levels were expected to subside.

Prime Minister Abhisit Vejjajiva warned, however, that new risks loom.

“We have to keep an eye on the situation closely,” Abhisit said Wednesday, adding that tidal levels were expected to rise again early next month. “The worrying period will return in the beginning of next month. The supervision must continue.”

Last week, Abhisit said the flooding was the country’s worst in 40 or 50 years due to heavy rains that dumped larger-than-normal amounts of water into dams and reservoirs and aggressive housing and business development that has affected natural drainage channels.

The Cabinet on Tuesday allocated 283 million baht ($9.42 million) to purchase boats, mobile toilets, sandbags, tents and other equipment to assist flood victims. About 600,000 households that suffered flooding for more than one week would be given initial relief funds of 5,000 baht ($170).

The Public Health Ministry has ordered 55,100 sets of mobile toilets in the flooded areas. The toilets vary from cardboard box toilets to floating toilets and toilets improvised from chairs and plastic bags.