Monkeys were cooling off in a concrete pool this week as temperatures in Thailand touched on a sweltering 40 degress Celsius amid the start of a summer heatwave.

Footage shows the primates leaping into cool liquid and sipping from the tap at the custom-built circular outdoor pool in Lopburi, around 100 miles from the capital Bangkok, on April 6.

The scenes came as medics raised fears that pets and humans could suffer from heat stroke in the hot weather, which reached 39 degrees Celsius in Nakhon Ratchasima province on Wednesday April 5.

Dr Opas Karnkawinpong, permanent secretary for public health, told local media that the intense summer heat could affect people's health. It could be especially dangerous for children, the elderly, and those with underlying ailments.

The government medical officer also warned heat stroke may develop if a person is exposed to heat for too long.

He caid that temperature readings taken in the sun would reach 40 degrees Celsius in Phetchabun and as high as 49.4 degrees Celsius in Chonburi province.

Officials said the highest temperature ever officially recorded in the country was 44.6 degrees Celsius in Mae Hong Son province on April 28, 2016.

April is the hottest month in Thailand before the country's annual rainy season arrives in May with daily tropical downpours.