City authorities in Bangkok gave an extreme heat warning as the heat index was expected to rise above 52 degrees Celsius (125 degrees Fahrenheit).
Temperatures in the concrete sprawl of the Thai capital hit 40.1 C on Wednesday and similar levels were forecast for Thursday.
A wave of exceptionally hot weather has blasted parts of South and Southeast Asia this week, prompting schools across the Philippines to suspend classes and worshippers in Bangladesh to pray for rain.
The heat index -- a measure of what the temperature feels like taking into account humidity, wind speed and other factors -- was at an "extremely dangerous" level in Bangkok, the city's environment department warned.
Authorities in Udon Thani province, in the kingdom's rural northeast, also warned of blazing temperatures on Thursday.
The health ministry said late Wednesday that 30 people had died from heatstroke between January 1 and April 17, compared with 37 in the whole of 2023.
Direk Khampaen, deputy director-general of Thailand's Department of Disease Control, told AFP that officials were urging elderly people and those with underlying medical conditions including obesity to stay indoors and drink water regularly.
April is typically the hottest time of the year in Thailand and other countries in Southeast Asia, but conditions this year have been exacerbated by the El Nino weather pattern.
Last year saw record levels of heat stress across the globe, with the United Nations weather and climate agency saying Asia was warming at a particularly rapid pace.
The kingdom has sweltered through a heatwave this week, with a temperature of 44.2 C recorded in the northern province of Lampang on Monday -- just shy of the all-time national record of 44.6 C hit last year.
Across the border in Myanmar, the temperature reached a blazing 45.9 C on Wednesday, with more of the same expected Thursday.
The chaos and conflict unleashed by the military's 2021 coup has led to rolling power blackouts in much of the country, hampering people's ability to keep cool with fans and air-conditioning.
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