Hackers target U.S.-South Korea joint military drills, but police confirm classified data is secure.

On Sunday, South Korean police reported that suspected North Korean hackers targeted the ongoing US-South Korea joint military exercises this week, although official confidential information was not compromised.

South Korean and United States troops will commence the 11-day Ulchi Freedom Guardian summer exercises on Monday, aimed at enhancing their ability to counter the increasing threat of North Korea's nuclear weapons and missiles.

North Korea opposes such exercises, claiming they are preparations for an invasion by the US and its South Korean ally.

According to a statement from the police station in the southern Gyeonggi region of South Korea, it is believed that the hackers are linked to North Korea's Kimsuky as claimed by researchers. They reportedly sent phishing emails to South Korean contractors working at the US-South Korea joint military exercise simulation center. The statement on Sunday confirmed that the relevant military information remained intact.

North Korea has previously denied involvement in any cyber-attacks. According to researchers, the Kimsuky hackers have long been using spear-phishing attacks to trick targets into leaking passwords or clicking on attachments or links to load malicious software.

South Korean police and the US military conducted a joint investigation and found that the IP addresses used in the hacking attack matched those identified in a 2014 hack against a South Korean nuclear reactor operator. At that time, South Korea accused North Korea of being behind the cyber-attack.