Seoul - South Korea's nuclear power operator said on Sunday that cyber attacks on non-critical operations at the company's headquarters are continuing, but the country's nuclear power plants are operating safely.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 cyber attacks or the company's response, citing security reasons."We cannot let cyber attacks 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."Cyber attacks on KHNP's (headquarters) operations and administration are still continuing now," he added.Systems hackedKHNP, 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 apologised for concerns that had been raised by the cyber attack 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 licence 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 against attempted cyber attacks on nuclear plants.'Destruction'Last week, a hacker demanded the shutdown of three reactors, also threatening "destruction" if the demand was not met via Twitter.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 cyber attacks, calling such suggestions part of a "smear campaign" by unpopular South Korean leaders. North Korea remains technically at war with the South.