Almost 65 years after the end of the Korean War, nearly every able-bodied South Korean man between the ages of 18 and 35 must still complete around two years of military service.

Anyone refusing the call-up has usually ended up in prison for 18 months, and with no alternative community service option around 19,000 conscientious objectors have been jailed since 1950, most of them Jehovah's Witnesses.

But the Supreme Court overturned the conviction of a conscientious objector on Thursday, months after a landmark constitutional court ruling calling for an alternative to military service for conscientious objectors.

In Thursday's case, a Jehovah's Witness identified only by his surname Oh was called to military service in 2013 but refused, was found guilty, and lost an initial appeal to the high court.

"Conscientious objection is a valid reason" to refuse conscription, the court said in its ruling, Yonhap news agency reported.

The decision overturned a previous Supreme Court ruling 14 years ago.

According to the Jehovah's Witnesses, more than 900 similar cases are currently pending in the South Korean justice system.