SEOUL - South Korean President Moon Jae-in on Friday ordered an investigation into the apparent suicide of a navy sergeant who had reported being sexually harassed by her supervisor, the second such case involving the military in recent months.

"I don't know how to convey my condolences to the bereaved family," Moon said, according to presidential spokeswoman Park Kyung-mee, as he tasked the Ministry of National Defense with the investigation.

Defence Minister Suh Wook apologised to the woman's family and the public, adding that his ministry would fully disclose its findings.

The female navy sergeant was found dead in her quarters on Thursday, the defence ministry said. She had reported being sexually harassed by her boss in a restaurant in late May, the ministry added, without providing details of the alleged offence.

The sergeant notified a superior at the time but did not officially report the incident until last week, when she directly advised a commanding officer.

Her death comes after the suicide of an Air Force master sergeant in May, who had accused a colleague of sexual abuse, sparked a national outcry that forced the Air Force chief to resign. 

The man charged in that case pled guilty on Friday to forcefully molesting the woman in March as they were returning to their base in a car after dinner in the western city of Seosan. In his first hearing at a military court, the man denied claims of blackmail, Yonhap news agency reported.

South Korea has in recent years been hit by a rash of sex crimes against women and children, including hidden-camera offences, "revenge porn" and online networks that blackmail women and underage girls into sharing sexual and sometimes violent images of themselves.

Sex crimes have also been reported in the military, which is one of the largest in the world with over 600,000 troops. Service in the military is mandatory for all able-bodied men.