North Korea hacked South Korea's cyber command in the latest cyberattack against Seoul, the South Korean military said.
"The military formed a cyber investigative team to look into this matter and found that some military data – including confidential information – has been leaked. It appears to be a North Korean act," the defence ministry said.
"It seems the intranet server of the cyber command has been contaminated with malware," an official at the Ministry of National Defense told Yonhap News Agency.
BBC reported that a spokesman said the classified information was thought to have been stolen but the extent of the infiltration is not yet clear.
Earlier, the North was accused of hacking into banks and media outlets but it was never related to the South's military. However, Pyongyang has rejected all the past allegations of cyber crime involvement.
The military said the affected network was isolated once the attack was detected to prevent the spread of the malware.
North Korea has thousands of cyberwarfare personnel and has a track record of waging cyberattacks on South Korean government agencies, banks and media companies and the United States in recent years.
North Korean defector and computer science professor Kim Heung-Kwang told the BBC that since 2010, they have been focusing on application programming interfaces (APIs), which can be designed to attack national infrastructure.
Reuters quoted police as saying that an apparent concerted campaign involving the planting of malicious code was started in 2014. According to police, nearly 140,000 computers at 160 companies were attacked up until this June.