A German tourist died after being stung by a poisonous box jellyfish off the popular island resort of Koh Samui, police said. Picture: Yasser Al-Zayyat

Bangkok -

A five-year-old French boy was killed by box jellyfish off an island resort in Thailand, prompting local authorities to issue a warning, local media said.

The boy died en route to hospital after being stung by jellyfish on Saturday while swimming off Koh Phangan in the Gulf of Thailand, the Daily News reported.

“We advise everyone to be cautious when swimming in the area,” Yongyos Thammawut of the local health department was quoted as saying.

Box jellyfish are relatively rare off the Thai coast, but their numbers swell in the summer months, and this year's population has grown particularly high, experts say.

Koh Phangan is known as home to the monthly Full Moon Party, as well as hosting a range of tourist resorts.

Box jellyfish are among the most venomous animals on earth. A sting from their tentacles, which can grow to 3 metres, can be fatal to humans, particularly children. - Sapa-dpa