The septet has become a global cultural phenomenon, selling out stadiums and dominating charts while raking in billions for South Korea's economy and building an international legion of fans known as ARMY.

But all able-bodied South Korean men must serve at least 18 months in the military, and after a years-long debate about whether BTS deserved an exemption, Jin, the oldest member of the group, began his service in December.

J-Hope -- whose full name is Jung Ho-seok -- was set to begin his five-week basic training at an army boot camp in Gangwon province on Tuesday, Yonhap News Agency reported.

The 29-year-old posted pictures of himself with a buzzcut on the South Korean social media platform Weverse on Monday.

"I love you, ARMY. I'll go and come back safely!" he said.

The post attracted thousands of comments from fans, with many expressing sorrow over the group's ongoing hiatus.

"Be careful and stay healthy... Say hi to Jin for us, please? We will miss you so much!" one wrote.

"It suits you so well," another wrote of J-Hope's buzzcut.

Jin left the comment "D-1" on one of J-Hope's Weverse posts on Monday -- a nod to the fact he had one day left before the start of his service.

The septet's agency HYBE had confirmed on April 1 that J-Hope would begin his service, but did not disclose details "to prevent any issues that might occur from crowding".

"The entrance ceremony is a time to be observed by military personnel and their families only," it had said.

"Fans are advised to refrain from visiting the site."

- 'Excited' -

J-Hope said during a livestream with fans last week that he was "excited" about his time in the military.

"It will be a new experience, and I will be learning to fit into a new society, and I'm excited about that," he said.

"In the meantime, what I look forward to is living each day in a normal way, which is different from my usual lifestyle. This could make me healthier."

The band had cited exhaustion and the pressures of stratospheric success while dropping their bombshell hiatus announcement last year, saying they were taking a break to focus on their solo careers.

Analysts have questioned what the future holds for the group after all seven members complete their military service.

Some male K-pop stars have struggled to resume their careers after military service in a cut-throat industry where artists are easily replaceable.

"During the absence, (BTS) could lose their public interest, and the decline in popularity will damage their business," Lee Taek-gwang, a communications professor at Kyung Hee University, told AFP.

"It would not be easy for the boy band to reunite... this is not only the problem of BTS but also the problem of almost all (South Korean) boy bands and male celebrities."