The state-run nuclear power plant in South Korea is reported to be preparing for cyber attacks from North Korea: Seoul has severed its internal network from the internet.

Korea Hydro & Nuclear Power, the only operator in South Korea, has said the network has been cut off in an effort to thwart any cyber attack launched by Pyongyang.

The control systems network at the plant has been divided and all connections to the internet have been disabled, said officials.

USB ports in computers and other systems have also been sealed off.

Insulating the networks is the best way to avoid any cyber attack, said power plant officials.

The country has 23 reactors, which produce around 35 percent of South Korea's electricity.

South Korea has been preparing for a massive cyber attack alongside the much-feared missile launch.

Seoul pointed fingers at North Korea over a recent cyber attack against the networks of major South Korean banks and broadcasters. Pyongyang denied any involvement in the attack.

"An analysis of cyber terror access logs, malicious code and North Korean intelligence showed that the attack methods were similar to those used by the North's Reconnaissance General Bureau, which has led hacking attacks against South Korea," Lee Seung-won, an official at the ministry of science, ICT & future planning, told a press conference.

The attack paralysed more than 40,000 computers at the three major financial firms and television broadcaster KBS, MBC and YTN.

Pyongyang, which has thousands of trained expert hackers, has been blamed for several previous cyber attacks on Seoul.