"I can open to the world 100,000 pages of data that have not yet been revealed," the hacker's tweet was quoted as saying by the agency.
The hacker has revealed data concerning the Gori-2 and Wolsong-1 nuclear reactors on his Twitter account called "president of anti-nuclear reactor group", according to the agency.
The data was reportedly leaked from South Korea's state-run Korea Hydro and Nuclear Power (KHNP) company and concerns the reactors' air conditioning and cooling systems. The hacker threatened to publish all the information that he had on the nuclear facilities if the reactors were not shut down by Christmas.
According to Yonhap, the latest publication of the KHNP materials is the fourth one since December 15. On December 19, the hacker demanded closing three other reactors for three months.
KHNP said that the information the hacker had did not pertain to the core technologies and did not pose a threat to the security of the reactors. KHNP operates 23 nuclear power reactors, providing 30 percent of South Korea's overall power supply, according to the company's website.