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1 year oldThe K-pop stars' label asked fans to continue giving support to RM, Jimin, V and Jung Kook until they return safely from conscription.
In South Korea nearly all able-bodied men are obliged to enlist, like the three older band members currently serving.
The group are due to reform in 2025.
"We would like to inform our fans that RM, Jimin, V, and Jung Kook have initiated the military enlistment process," label Big Hit Music said in a statement.
"We ask you for your continued love and support."
It comes after older band member Jin enlisted last year, followed by his bandmate J-Hope in April.
A third member Suga started service as a social service agent, after being "ruled unfit for the regular combat duty".
BTS went on a hiatus in June to allow members to complete military service after earning six number one singles in the US.
Previously there had been years-long debate on whether BTS deserved an exemption, on the basis they generated billions of dollars for the country and boosted the image of South Korea.
Earlier this year, chairman of the seven-man band's agency said making BTS do their military service was slowing the global growth of K-pop.
South Korea offers exemptions from military service to some people, including Olympic medal-winning athletes and some classical musicians, but not K-pop stars.
In September it was confirmed all seven members of the group had renewed their contracts with their music label.
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