A banner showing members of South Korean K-pop band BTS is displayed on a street in Seoul, South Korea, Dec 11, 2023. (PHOTO / AP)

SEOUL/TOKYO - The remaining four members of K-pop supergroup BTS will begin their 18-month military service in South Korea this week, joining three others already serving, with fans pledging to wait for them to perform again as a group until 2025.

A group of fans braved rainy weather on Monday to bid a temporary farewell to the seven-member band's leader, RM, and vocalist V in front of their boot camp in the central city of Nonsan.

ALSO READ: K-pop star and BTS member J-Hope starts military service

"I think I'll be fine except for one thing, that I won't be able to make happy memories with you for a while, which is the hardest part," V wrote on fan platform Weverse on Monday, wishing fans well and happiness.

RM also wrote that he might be lonely at times, but that the 18 months would be a chance to get new inspiration and learn new things.

In this May 21, 2021, members of South Korean K-pop band BTS pose for photographers ahead of a press conference to introduce their new single "Butter" in Seoul, South Korea. (Lee Jin-man / AP)

Since announcing a break from group projects in June 2022, BTS members have pursued solo activities before starting military service

The duo were accompanied by other members of the band, including Jimin and Jung Kook who will start their military service on Tuesday, and the three others who were given a day off from military duty, the Yonhap news agency said.

All able-bodied South Korean men ages 18-28 must serve in the military for about two years.

Known as ARMY, BTS’ global fan base of millions of loyal followers has helped the band support social causes. In 2020, supporters raised more than $1 million for the Black Lives Matter movement in just 25 hours.

Some ARMY fans gathered at a cafe in a Tokyo district known as Korea Town and vowed to wait for their return.

"I feel sad but I think they'll mature more going into the army and come back looking cool so I'm looking forward and want to wait for them," said Ayami Ito, 22, a nursing care worker.

ALSO READ: Media: Four BTS members to begin military service mid-Dec

Since announcing a break from group projects in June 2022, BTS members have pursued solo activities before starting military service.

Jin, 30, the oldest member, joined the army last December, followed by j-hope in April and Suga in August.

BTS' global success has triggered debate over whether K-pop artists should be allowed to serve for a shorter term, a perk granted to Olympics and Asian Games medallists and some classical musicians with international achievements.