South Korea's top court ruled Thursday that South Korean men can legally reject their mandatory military service on conscientious or religious grounds without punishment.

The landmark ruling is expected to affect the cases of more than 930 conscientious objectors on trial. Hundreds of young South Korean men, mostly Jehovah's Witnesses, are imprisoned every year for refusing to serve in the military.

All able-bodied South Korean men must serve about two years in the military under a conscription system aimed at coping with potential aggression from North Korea. The court broke with its own 2004 verdict that rejecting military service because of religious faith was illegal, saying at the time that confrontation with the North made South Korea's draft an indisputable necessity.

The ruling was great news for Jehovah's Witnesses and others who call for improved individual rights and freedom of opinion in South Korea. But many conservatives are likely to criticize it, saying it inadequately considers the North Korean threat.

When South Korea's Constitutional Court ruled in June that the government must provide alternative social service for conscientious objectors by 2019, a heated debate erupted over whether it is the proper time for such a measure because North Korea's nuclear threat remains unchanged. There are also worries that some might exploit alternative service to evade the draft.

On Thursday, the Supreme Court said it quashed a lower court's sentencing of a conscientious objector to 18 months in prison. It said it ordered the lower court to review its earlier verdict. Supreme Court officials said there is little chance the lower court would not abide by the decision.

The majority opinion of a panel of Supreme Court judges is that "conscientious objection of military duty ... can be a valid reason" to avoid military service, the top court said in a statement.