South Korea has registered its highest temperature since records began more than a century ago, as authorities warned people to “immediately” stop all outdoor activities in affected areas.

The south-eastern city of Yangsan saw the temperature reach 42.5C on Sunday afternoon, the Korea Meteorological Administration (KMA) said, the highest temperature reported since modern weather observation began in 1904.

Yangsan has been the centre of an ongoing heatwave, where temperatures exceeded 40C for a fifth consecutive day on Sunday, while many other cities including Daegu and Busan are seeing temperatures climb into the high 30s.

People gathered in airconditioned shopping malls or cafes to escape the blistering temperatures. Others sought relief under mist-spray zones set up in Seoul. Ju Dong-yeol, 27, took refuge from the heat in a Starbucks in the South Korean capital, along with his parents and told AFP “it is too hot to be outside doing anything”.

“I can’t think of anything else to do other than stay in airconditioned places like home, shopping malls and cafes,” the office worker said.

The city of Gyeongju, about 240km south-east of Seoul, reached 40.3C on Sunday, its highest recorded temperature in eight years.

North Korea has also faced punishing heat, with temperatures soaring as high as 40C in some regions, state media reported on Monday. Residents in Pyongyang have endured a week of nights where the temperature has not dropped below 25C.

More than 20 localities in South Korea were under emergency heatwave alerts on Sunday – a new warning category introduced this year to better address rising temperatures. An emergency alert is issued when areas experiencing a heatwave are forecast to hit perceived temperatures of 38C or an actual temperature of 39C for the day.

Workers harvest strawberries under parasols to protect against the scorching sun in Miryang, South Gyeongsang Province, South Korea. Photograph: YONHAP/EPA

Issuing a heat warning, the weather agency urged people to “immediately stop all outdoor activities”, adding that “indoor spaces without air conditioning are dangerous”.

“Move immediately to a cool place, such as a designated cooling centre or a shaded area, and stay hydrated while resting,” it advised.

Fearing for those people most vulnerable to heat, the prime minister, Han Seong-sook, ordered local officials to strengthen safety checks on residents and to ensure a stable power and water supply was maintained.

Easterly winds are expected to push the extreme heat further inland on Monday, with Seoul forecast to reach 37C. “Temperatures in South Jeolla and South Gyeongsang are expected to exceed 39C, bringing extreme heat at life-threatening levels,” the KMA said.

People stand in a mist-spraying zone to cool off on a street in Seoul. Photograph: Jung Yeon-Je/AFP/Getty Images

Officials are also monitoring Typhoon Dolphin, which is now tracking towards China’s south-east coastal regions, south of the Korean peninsula, and could dictate how long the stifling heat persists.

If the typhoon moves towards eastern China, it could increase humidity and potentially worsening the heatwave and tropical nights in Korea, the KMA said. But a path closer to the south of the peninsula could bring stronger winds, greater cloud cover and rain, temporarily easing the heat.

KMA data show the average annual number of heatwave days in the country has more than doubled to 19 in the past five years.

A heatwave day is defined as one with a maximum temperature of at least 33C.

South Korea’s professional baseball league was forced to cancel one game in Changwon after the city’s temperature climbed to 39.5C, marking the second consecutive game cancellation in the city.

“The decision was made with the safety and health of spectators and players as the top priority, as a severe heatwave warning remains in effect for the Changwon area,” said the Korea Baseball Organization in a statement.

Scientists warn that extreme weather events such as heatwaves are becoming more frequent and intense as a result of human-induced climate change.

Adding to the extreme weather is the return this year of El Niño – a natural climate phenomenon that warms Pacific Ocean surface temperatures and typically occurs every two to seven years.

With Agence France-Presse