Japan has told the government that it would seek Seoul's approval before sending troops to the Korean Peninsula in an emergency.
Defense Ministry spokesman Kim Min-seok told reporters on Monday that Korea had told Japan it needs South Korean approval before sending troops to the Korean Peninsula and that the Japanese government "made it clear" that it would not do so unilaterally.
Japan is keen to assert its right to so-called collective self-defense, a euphemism for military intervention abroad if an ally is in some way under threat.
The government informed Japan of this requirement during a meeting in Washington between senior defense officials from South Korea, the U.S. and Japan last week.
A Defense Ministry official said this could happen if there is an emergency in North Korea or the North attacks Japan.
Koreans are wary of any Japanese military presence on the peninsula after Japan's brutal occupation from 1910 to 1945.