SEOUL (REUTERS) - A recent series of leaks of data from South Korea's nuclear operator was a "grave situation" that was unacceptable as a matter of national security, President Park Geun Hye said on Tuesday.

"Nuclear power plants are first-class security installations that directly impact the safety of the people," Park said at a cabinet meeting, according to her office.

"A grave situation that is unacceptable has developed when there should have been not a trace of lapse as a matter of national security," she said.

She ordered inspections of safeguards at national infrastructure facilities, including the nuclear power plants, against what she called "cyber terrorism."

Earlier, a South Korean official said Seoul had not ruled out the possible involvement of North Korea in the cyberattack on South Korea's nuclear power plant operator, although Park did not make any mention of it.

Korea Hydro and Nuclear Power Co Ltd (KHNP), which runs South Korea's 23 nuclear power reactors, said on Monday that its computer systems had been hacked, raising alarm in a country that remains at war with North Korea.

The nuclear operator said only non-critical data had been stolen and operations of the nuclear plants were not at risk.

South Korea's law enforcement authorities have opened an investigation.

The incident came after the United States accused North Korea of a devastating cyberattack on Sony Pictures and vowed to respond proportionately.