Citing Seoul’s defense ministry, Rhee said that 80 percent of the leaked documents had yet to be identified.
But the contingency plan for the South’s special forces was stolen, he said, as well as details about annual joint military drills with the US and information on key military facilities and power plants.
A ministry spokesman declined to confirm the report, citing intelligence matters.
In May, the ministry said North Korea had hacked into Seoul’s military intranet but did not say what had been leaked.
Pyongyang has a 6,800-strong unit of trained cyberwarfare specialists, according to the South Korean government. It has been accused of launching high-profile cyberattacks, including the 2014 hacking of Sony Pictures.
The Chosun Ilbo story was the second report Tuesday of military-related cyber-attacks in the Asia-Pacific.
Australia’s government said separately an unidentified defense contractor had been hacked and a “significant amount of data” stolen.
There were 47,000 cyber-incidents in the last 12 months, a 15 percent increase from the previous year, Minister for Cyber Security Dan Tehan said in Canberra as he unveiled a report by the Cyber Security Center.
The defense contractor was exploited via an Internet-facing server, with the cyber-criminals using remote administrative access to remain in its network, the report said.
The hacker was reportedly based in China, but Tehan told the Australian Broadcasting Corporation that “we don’t know and we cannot confirm exactly who the actor was.”