The US Federal Bureau of Investigation (FBI) and the president have made clear that the North Korean government was responsible for the attack, said deputy spokesperson for US State Department, Marie Harf, according to a media report.

"We stand by this conclusion," she said.

The FBI claimed on Friday that it had "enough information" to conclude that North Korea was responsible for hacking into Sony computers and posting online some of the stolen data in late November.

Sony last week cancelled the planned Christmas release of its comedy movie "The Interview," which depicts a fictional assassination attempt against the North Korean leader Kim Jong-un, after major US cinema chains decided not to screen the movie as hackers warned movie-goers to stay away from cinemas showing the film.

The North Korean government was outraged by the film's storyline, revolving around a fictitious US plot to assassinate the North Korean leader Kim Jong-un.  

It claimed to have "clear evidence" that the US government engineered the project as a "propaganda" attack against the country, according to media reports. While speaking at his end-of-the-year news conference on Friday, US President Barack Obama had said that Sony made a mistake by cancelling the release of the movie.

He also said that he wished the company had contacted him before taking the action, vowing to "respond proportionally" to the cyber attack.

North Korea has flatly rejected the US accusations and proposed a joint investigation US.

Harf said that, if North Korea wanted to help, "they can admit their culpability and compensate Sony for the damages that they caused".

"We do urge North Korea to exercise restraint (and) to refrain from further threatening actions," she added.

Harf declined to disclose what US retaliatory measures would be, reiterating that US would implement its response.

"Some will be seen, some may not be seen," she said.  

The US government has said that it stands by its accusation and will respond "proportionately".

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