Extreme heat has blasted Asia from India to the Philippines in recent weeks, triggering heatstroke deaths, school closures and desperate prayers for cooling rain.

Scientists have long warned that human-induced climate change will produce more frequent, longer and intense heatwaves.

Vietnam saw three waves of high temperatures in April, according to data published Friday by the National Centre for Hydro-Meteorological Forecasting, with the mercury peaking at 44 degrees Celsius (111.2 Fahrenheit) in two towns earlier this week.

The mark is only slightly below the highest temperature ever recorded in Vietnam -- 44.2 C on May 7 last year.

In all, 102 weather stations saw record highs in April, as northern and central Vietnam bore the brunt of the heatwave, with temperatures on average 2-4 C higher than during the same period last year.

Seven stations recorded temperatures above 43 C, all on Tuesday.

The most dramatic sign of the extreme weather hitting Vietnam came in the southern province of Dong Nai, where hundreds of thousands of fish died in a reservoir.

Images showed locals wading and boating through the 300-hectare Song May reservoir, with the water barely visible beneath a blanket of dead fish.

The mass die-off was blamed on water shortages caused by the heatwave and poor management.

The Vietnamese weather agency is predicting more hot weather in May, with temperatures expected to be 1.5 to 2.5 degrees higher than in previous years.

While April and May are normally the hottest time of year in Southeast Asia, experts say the El Nino effect is making this year's heat particularly intense.

Bangladesh and Myanmar saw April heat records broken, heatstroke has killed at least 30 people in Thailand since the start of the year, and high temperatures were partly blamed for a deadly explosion at a Cambodian ammunition dump.

Roman Catholic bishops in the Philippines are urging the faithful to pray for rain and lower temperatures, after the heat forced the government to close tens of thousands of schools.

The Indian megacity of Kolkata has sweltered through punishing heat, peaking at 43 C for the city's hottest single April day since 1954.

Even mountainous Nepal has been hit, with the government issuing health warnings last week and firefighters battling unusually severe wildfires.