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

But all able-bodied men in South Korea 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, enlisted 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 on Weverse late Monday, with his newly shorn, military-approved buzzcut.

"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. ARMY will do its best to stay positive by streaming and supporting the things you love and enjoy! 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. "With much respect, I salute you. For your loyalty, honour, and sense of duty to serve your country."

Band member Jin, who is currently serving in the military, 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 enlistment.

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

"Please note that there will be no official event on the day of his entry," the agency said at the time.

"The entrance ceremony is a time to be observed by military personnel and their families only... Fans are advised to refrain from visiting the site."