SEOUL (Reuters) - South Korea’s nuclear power operator said on Sunday that cyberattacks on non-critical operations at the company’s headquarters are continuing but the country’s nuclear power plants are operating safely and are secure from attack.

Korea Hydro & Nuclear Power Co Ltd has been intensifying its cyber security, President and CEO Cho Seok said. He gave no details of the continued cyberattacks or the company’s response, citing security reasons.

“We cannot let cyberattacks stop nuclear power operation,” Cho told a news briefing. He added that a closed network used for reactor operations was inaccessible from external communication lines and impervious to cyberattacks.

“We will continue operating nuclear plants safely against any attempted foul play, including cyberattacks,” Cho said.

“Cyberattacks on KHNP’s (headquarters) operations and administration are still continuing now,” he added.

KHNP, part of state-run utility Korea Electric Power Corp, said last Monday that its computer systems had been hacked but only non-critical data had been stolen and reactor operations were not at risk.

Cho apologized for concerns that had been raised by the cyberattack and data leaks but said the nuclear plants themselves had not been affected. South Korea has 23 nuclear reactors which supply one-third of its electricity. Three are currently offline for routine maintenance or awaiting a license extension.

The operator and the government since last Wednesday have been running emergency teams on stand-by until the end of the year as a precaution in case of any attempted cyberattacks on nuclear plants, after a hacker demanded the shutdown of three reactors by last Thursday and in Twitter messages threatened “destruction” if the demand was not met.

South Korean prosecutors are also seeking the cooperation of Chinese authorities in an investigation into the cyberattack, after tracing multiple Internet addresses to a Chinese city near North Korea. They have not ruled out possible involvement of North Korea in the attack.

Pyongyang denied any role in the cyberattacks, calling such suggestions part of a “smear campaign” by unpopular South Korean leaders. North Korea remains technically at war with the South.