The mercury was forecast to hit 39 C in the sprawling Thai capital, while the heat index rose above 52 C, classed as "extremely dangerous" by city authorities.

The heat index is a measure of what the temperature feels like, taking humidity, wind speed and other factors into account.

"Warning: the heat index today is 'extremely dangerous'. Please avoid activities outdoors," the Bangkok city authority's environment department wrote in a Facebook post.

"The heat index is classed as extremely dangerous when it goes above 52 degrees Celsius."

April is typically the hottest and most humid time of the year in Thailand, but conditions this year have been exacerbated by the El Nino weather pattern.

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.

The hot weather is expected to continue in the coming days, and the government has urged people to stay hydrated, apply sunscreen and exercise indoors to avoid heatstroke.

Global temperatures hit record highs last year, and the United Nations weather and climate agency said Tuesday that Asia was warming at a particularly rapid pace.

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