Japan did not announce an anti-piracy drill it conducted with South Korea in the Gulf of Aden off Somalia in late December due to concerns that doing so could impede bilateral negotiations to resolve the so-called "comfort women" issue, a senior Japanese government official said Wednesday.

The Japanese Maritime Self-Defense Force and the Republic of Korea Navy conducted the exercise on Dec. 23, five days before the foreign ministers of the two countries struck a landmark agreement to "finally and irreversibly" resolve the issue of Korean women forced into wartime Japanese brothels, the official said, requesting anonymity.

The official cited mixed feelings among the South Korean people as to whether Seoul should promote defense exchanges with Tokyo. Japan ruled the Korean Peninsula as a colony from 1910 to 1945.