Making megastars BTS do their South Korean military service is slowing the global growth of K-pop, the chairman of the septet's agency HYBE said Wednesday.

The boyband are widely considered South Korea's biggest-ever cultural phenomenon, selling out stadiums around the world and dominating the charts while raking in billions for the country's economy 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, after a years-long debate about whether BTS deserved an exemption, Jin, the oldest member of the group, enlisted in December.

Bang Si-hyuk, chairman of HYBE and the mastermind behind BTS, said figures -- including global ratings and album sales -- now showed declining demand for K-pop.

"The absence of BTS is the first reason," Bang said.

"I think it's very clear that the fact BTS is not active as a team is playing the biggest role in bringing about this change in numbers," he told a forum in Seoul.

BTS are the most prominent K-pop figures globally, he said, adding: "the market narrows significantly when BTS is taken out."

His comments come days after HYBE suspended a controversial bid to acquire control of rival SM Entertainment, ending a lengthy, high-profile feud over the company's management.

He was accused of a hostile takeover and seeking to monopolise K-pop, but Bang defended his move as a solution to K-pop's slowing growth.

- J-Hope next -

HYBE announced last month that a second BTS member, J-Hope, has "initiated the military enlistment process", with local media reporting he was due to enlist in April.

On Sunday, J-Hope appeared on a late night music talk show on KBS and said goodbye to fans, telling them: "I will be back soon from the military."

Since their debut in 2013, BTS have been credited with generating billions for their country, as well as boosting the image and soft power of South Korea -- now a global cultural powerhouse.

They have been invited to speak at the United Nations, and to meet US President Joe Biden at the White House.

But 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.

"Obviously, I think it's true that enlisting in the military means the continuity of a person's career or their value as a national asset is damaged," said HYBE chairman Bang.

"However, I think it's a completely different matter for an individual to happily accept the duty of the military," he added, saying he was "happy" about the decisions by the BTS members.