Hundreds of participants at the 25th World Scout Jamboree in Buan, South Korea have been hit by heat exhaustion.

Some 400 cases were reported on the first night of the event on Tuesday, with many treated at a makeshift hospital at the campsite.

Temperatures have hit 35C (95F) in North Jeolla province, where the event is being held, amid a heatwave warning.

The annual jamboree, sometimes called the world’s largest youth camp, kicked off on Tuesday with some 43,000 Scouts.

While participants are typically aged from 14 to 18, the event also draws many adults such as celebrity survivalist Bear Grylls, 49, who is the United Kingdom’s Chief Scout.

A fire service official told Reuters that most of the Scouts who required medical attention experienced mild symptoms such as headaches, dizziness and nausea.

The official added that most have since returned to their camp sites.

Scouts from countries such as the United States, Britain and Bangladesh were affected, said local media reports.

South Korea’s interior minister Lee Sang-min on Thursday urged jamboree organisers to adjust their programmes according to the number and severity of cases.

Choi Chang-haeng, secretary general of the event’s organising committee, told a press conference on Thursday that 39 people are still being treated at medical institutions.

He added that additional medical personnel have been deployed at the event, and cooling devices are on-site to ensure the event can proceed smoothly.

Parts of South Korea have been roasting in an unusually hot summer.

Earlier this week, Seoul authorities raised its hot-weather warning to the highest level for the first time in four years, as temperatures hovered between 33C and 38C.

The scorching heat is estimated to have killed at least 16 people across the country, more than doubling the record of seven during the same period last year.