South Korea steps up cybersecurity at nuclear plants

South Korea boosted cybersecurity at the country's nuclear power plants on Tuesday following what President Park Geun-hye described as a series "grave" data leaks, and as prosecutors said they were investigating a new online threat.

Korea Hydro and Nuclear Power Co. (KHNP), which runs South Korea's 23 nuclear power reactors, said on Monday its computer systems had been hacked, raising alarm in a country that is still technically at war with North Korea, amid signs that Pyongyang is mounting a growing cyberoffensive with international adversaries to the south and west.

Seoul on Tuesday raised the cybercrisis alert by one level for all the state-run companies, from attention to caution.

The nuclear operator, part of state-run utility Korea Electric Power Corp, said only noncritical data were stolen and operations of the nuclear plants were not at risk. South Korea's law enforcement authorities are investigating the leaks.

"Nuclear power plants are first-class security installations that directly impact the safety of the people," Park said at a Cabinet meeting, according to her office.

"A grave situation that is unacceptable has developed when there should have been not a trace of lapse as a matter of national security," she said.

Within hours of Park's comments, an online user who claimed to have hacked the nuclear operator posted a new threat and a fresh batch of data on the same Twitter account that was used for previous threats and leaks.

"We are now looking at it ... We believe it was done by the same user," an official at South Korean prosecutors' office investigating the leaks told Reuters.

An official at the nuclear operator said it was working to verify whether the data were taken from its computers.

Earlier, the investigation team official said Seoul had not ruled out the possibility that North Korea was involved in the cyberattack, although Park did not make any mention of it.

The official added that South Korea had requested Washington's help investigating the matter.

Seoul is working to addresses cybersecurity breaches amid what appears to be simmering cyberdispute between its U.S. ally and Pyongyang. The FBI accuses North Korea of being behind the hack of Sony Pictures Entertainment’s server that led to the release of embarrassing company emails and caused Sony executives to halt the debut of the movie "The Interview," about a fictional plot to kill North Korean Supreme Leader Kim Jong Un.

In a sign of mounting cyberwarfare between Washington and Pyongyang, North Korea experienced sweeping Internet outages this week, and one computer expert said the country's online access went completely dark. The Associated Press reported on Tuesday that after 10 hours of outages, key North Korean websites were back online.

Al Jazeera and wire services