The nuclear power plant operator in South Korea began a two-day drill after cyber-attacks by an alleged hacker leaked information from the company on Twitter, according to The Guardian.

The test was started on Monday at each of South Korea's four nuclear plants. Korea Hydro and Nuclear Power (KHNP) operates 23 reactors that supply about 30 percent of the country's electricity. "The two-day drill is under way through simulators to ensure the safety of our nuclear power plants under cyber-attacks," said Kim Tae-Seok, KHNP spokesman, according to The Guardian.

Last week, a suspected hacker published on Twitter reactor designs and personal information on 10,000 KHNP workers. The hacker released a message on Sunday threatening more leaks unless South Korea shuts down three of its reactors.

Deputy Energy Minister Lee Kwan-Sup stated the leaked information seemed to be from the plants in the south-east of Seoul, according to The Guardian.

The safety drills in response to the cyber-attacks follow the confirmation by U.S. President Barack Obama that North Korea is responsible for the hacks at Sony Pictures.

The Federal Bureau of Investigation says there is "significant overlap" between attacks of "malicious cyber activity" with direct ties to Pyongyang (the capital of North Korea), including an attack on South Korean banks, according to The Guardian.

North Korea has denied any involvement with the hacks, but has threatened strikes against the United States (the White House and other landmarks specifically) if the U.S. government resorts to sanctions against North Korea.