North Korea has threatened retaliatory action against the United States after the FBI reported on Friday that North Korea was responsible for the hack on Sony Pictures, which resulted in the release of emails and employee information.

US President Barack Obama said he would respond "proportionately" to the incident, which he described as "cybervandalism" and not "an act of war" in an interview that aired on CNN yesterday.

Sony decided last week to withdraw 'The Interview' film that sparked the current tensions. The comedy portrays a fictional plot to assassinate North Korean leader Kim Jong-Un, and was scheduled for release on Christmas Day.

"We do not know who or where they [the hackers] are but we can surely say that they are supporters and sympathizers with the DPRK [Democratic People's Republic of Korea]," said the country's Policy Department of the National Defence Commission in a long statement on KCNA, North Korea's state news agency.

The statement described the hack as "righteous action," backing it up with threats against the US.

"Our toughest counteraction will be boldly taken against the White House, the Pentagon and the whole U.S. mainland, the cesspool of terrorism, by far surpassing the 'symmetric counteraction' declared by Obama," the statement said.

Obama has said he is considering whether to put North Korea back on a list of state sponsors of terrorism, from which it was removed six years ago.

This is the first time the United States had directly accused another country of a cyberattack of such magnitude on American soil and has established the possibility of a new confrontation between Washington and Pyongyang.

Republican Senator John McCain disagreed with Obama's description of the attack, and told CNN that it was the manifestation of a new kind of warfare.

On Friday the President said it was a "mistake" for Sony to withdraw the film and give in to blackmail. However Sony Pictures Entertainment CEO Michael Lynton defended the move, saying that US cinemas did not want to show the film.

China, North Korea's main ally, has said it opposes all forms of cyberattacks, but has said there is no proof that North Korea is responsible for hacking Sony.

Chinese Foreign Minister Wang Yi "reaffirmed China's relevant position, emphasizing China opposes all forms of cyberattacks and cyber terrorism" in a conversation with US Secretary of State John Kerry on Sunday, the foreign ministry in Beijing said in a statement.

"[China] opposes any country or individual using other countries' domestic facilities to conduct cyberattacks on third-party nations," it said.

The statement made no direct mention of North Korea.

The US has also accused China of cyber spying in the past and a US official has said the attack on Sony could have used Chinese servers to mask its origin.

South Korea, which is still technically at war with North Korea, said today that computer systems at its nuclear plant operator had been hacked and non-critical data stolen, but there was no risk to nuclear installations or reactors.

"It's our judgment that the control system itself is designed in such a way and there is no risk whatsoever," Chung Yang-ho, deputy energy minister, told Reuters.

He made no mention of North Korea and could not verify messages posted by a Twitter user claiming responsibility for the attacks and demanding the shutdown of three aging nuclear reactors by Thursday.

Japan, one of Washington's closest allies in Asia, said it strongly condemned the attack on Sony Pictures, but also stopped short of blaming North Korea.

"Japan is maintaining close contact with the United States and supporting their handling of this case," Chief Cabinet Secretary Yoshihide Suga said at a news conference.

(Additional reporting by Reuters)