The North hacked into smartphones belonging to dozens of major South Korean officials between late February and early March and gained access to lists of phone conversations, the content of text messages and phone conversations, the National Intelligence Service said.
North Korean hackers also sent text messages to South Korean officials to try to lure them into following links to malicious software, it said.
The spy agency said about one out of five smartphones were infected with malware, which led to the leak of the phone numbers of many other senior South Korean officials.
The latest cyberattacks came amid heightened tensions on the Korean Peninsula following Pyongyang's nuclear test and long-range rocket launch earlier this year.
North Korea has a track record of waging cyberattacks on South Korea and the United States in recent years, though it has flatly denied any involvement.
South Korea and the U.S. began the largest-ever annual military drill on Monday in the latest show of force against North Korea. In response, North Korea threatened to counter with a preemptive attack. (Yonhap)