SEOUL (Yonhap) -- South Korea said Thursday that a recent series of cyberattacks on its nuclear power operator would not cause any direct danger to the operation of 23 nuclear reactors in the country.

Over the past week, an anonymous anti-nuclear group had posted a series of documents and operating manuals for some South Korean nuclear reactors on the Internet, causing a safety concern in the country.

But the Korea Hydro & Nuclear Power Co., the operator of 23 nuclear reactors in the country, claimed the documents and information posted on the Internet were not classified and would not cause troubles in operating nuclear plants.

The South Korean government has been on an emergency stand-by since Monday as the self-proclaimed anti-nuke group demanded a halt to the operation of three nuclear reactors by Thursday and threatened to make public more documents unless the government shuts down three reactors at Gori and Wolsong in southern South Korea.

The presidential office, Cheong Wa Dae, said all countermeasures have been taken to safeguard the nuclear reactors.

"The control system of nuclear reactors are separated from external networks, and hacking into the system is fundamentally impossible," the presidential office said in a statement after a meeting chaired by National Security Adviser Kim Kwan-jin.

On Wednesday, a joint investigation team of government and prosecution officials said a suspect in the hacking attack was found to have used multiple Internet protocol (IP) addresses based in China.

South Korean investigators asked Chinese authorities to help identify the suspected IP addresses the hackers used in the Chinese city of Shenyang, which is close to North Korea.

South Korean officials have not ruled out the possible involvement of North Korea, which has a track record of waging cyber attacks on major financial institutions, government web sites and media organizations in South Korea. North Korea has flatly denied its involvement.