According to the news agency, the attack was detected on May 14. The hackers' IP addresses appeared to be linked to a hacking unit, known as 'kimsuky', allegedly belonging to Pyongyang's Reconnaissance General Bureau, Nikkei Asia said.
"Government agencies are currently assessing the damage," KAERI was cited as saying by Nikkei.
The incident was reported by a member of the South Korean National Assembly's Intelligence Committee, from the opposition People Power Party.
He claimed that the hackers could have stolen nuclear power technology during the attack and warned that, if this was the case, the damage "would be second only to the 2016 attack on our national defense network," Nikkei stated.
Pyongyang denied its involvement in the hacker attacks on South Korean Defense Ministry servers in 2016.