The Southeast Asian nation is the world's second-biggest exporter of rice, some grown in its northeast. It is in the middle of the annual rainy season and floods, which began on July 5, have been unusually heavy, authorities said.
Ten of Thailand's 77 provinces are disaster zones, the Interior Ministry said, adding that most of the 700,000 hectares (1.7 million acres) affected were rice-growing farmland.
Floods in Thailand's northeast kill 23
"Losses from the floods are estimated at at least 10 billion baht," or the equivalent of $300 million, Jirapan Assawathanakul, chief of the Thai General Insurance Association, told reporters.
The floods had not affected rice exports but it was too early to assess damage as the crop can survive short-term inundation, Chookiat Ophaswongse, president of the Thai Rice Exporters Association, has said.
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In 2011, widespread floods killed more than 900 people and caused major industrial disruption, cutting economic growth to just 0.1 percent.
When Thailand's ruling junta took power in 2014, it proposed a 10-year water management plan to avoid a repeat of the 2011 floods, but it is still under review.
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