South Korea’s sole nuclear plant operator Korea Hydro and Nuclear Power’s (KHNP) computer systems have been hacked, raising concerns over the safety of nuclear power plants in the country.

Estimates of radiation exposure, data on blueprints of reactors, designs of equipment and internal manuals of the KHNP were hacked and posted online last week by a hacker.

The identity of this hacker, whose account was named ‘president of the anti-nuclear reactor group’, is as yet unknown.

A warning was also given by the hacker to stay away from the nuclear reactors, and demanded that these facilities be closed by Christmas, reports UPI.

"A compromise of nuclear reactors’ safety pretty clearly means there is a gaping hole in national security."

The KHNP and the government, however, clarified that only the non-critical was breached and nuclear reactors do not face any risk.

Experts believe the controls of these reactors could be in danger, reports Reuters.

The breach has prompted a safety drill at nuclear facilities across around the country.

The precautionary measure comes only a few days after the US accused North Korea of hacking Sony Pictures.

Nuclear reactor design expert Suh Kune-yull of Seoul National University was quoted by Reuters as saying: "This demonstrated that, if anyone is intent with malice to infiltrate the system, it would be impossible to say with confidence that such an effort would be blocked completely.

"A compromise of nuclear reactors’ safety pretty clearly means there is a gaping hole in national security."

There are concerns that the hackers could possibly have links with North Korea, and may focus on crucial infrastructure.

The government is looking into the incident, but so far has not blamed any one directly.

In 2013, South Korea blamed its northern counterpart for launching cyber attacks on banks and broadcasters.

South Korea’s energy ministry expressed confidence that its nuclear facilities could thwart any breaches that compromise the reactors’ safety.

Deputy energy minister Chung Yang-ho told Reuters: "It’s our judgement that the control system itself is designed in such a way and there is no risk whatsoever."

KHNP, which is part of the state-run utility Korea Electric Power Corp, operates 23 reactors in the country. These cater to approximately one-third of the country’s energy requirements.