BTS star Jin starts his mandatory South Korean military duty on Tuesday, the band's first member to enlist since a hiatus announcement this year left fans heartbroken over the K-pop juggernaut's uncertain future.

The septet is widely considered the country's biggest-ever cultural phenomenon – selling out stadiums around the world and dominating the charts while raking in billions and building a global legion of fans known as ARMYs.

But all able-bodied men in South Korea must serve at least 18 months in the military, and while there was a years-long debate about whether BTS deserved an exemption, they confirmed in October that all members will enlist.

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Jin – whose full name is Kim Seok-jin – will begin five weeks of training on Tuesday, the military said.

Yonhap news agency reported the star is expected to be deployed to a "frontline" unit near the border with North Korea, with which South Korea remains technically at war.

"We ask you to keep your heart-warming words of support and farewell in your hearts," BTS's label Big Hit Music told fans last week, urging them to stay away from the families-only entrance ceremony.

Fans were stunned in June when the group revealed that it was going on hiatus, citing exhaustion and pressure as well as the desire to pursue solo careers.

But analysts said the announcement was strategically timed because of the compulsory military duty.

The group will reunite around 2025, when its seven members have completed their service.

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"For a while, it's true that there were many fans who would spend days just crying," a South Korean fan, who runs the Twitter account @5heterotopia, told AFP.

Nimah Mustafa, a 20-year-old fan in Dubai, added: "(Jin's absence) will be like a huge... void for me."

South Korea exempts some elite athletes, such as Olympic medallists, and classical musicians from duty, but pop stars do not qualify.