South Korea's monopoly nuclear power company says it is conducting drills against possible cyberattacks after online threats of attack against its plants.

State-owned Korea Hydro & Nuclear Power Co. said Monday that the two-day drills are to prepare workers in the event of hacking attacks aimed at disabling the plants' controlling systems.

The company and the energy ministry said even though the controlling system is safe from hacking, they are holding the exercises to assure public of safety.

Last week, documents including layouts of nuclear facilities were published online. The company said they were not confidential.

On Sunday, a user on Twitter demanded a halt to operations at three nuclear power plants. The user threatened to disclose more data and carry out another attack if the request was not met.