James Franco and Seth Rogen at the premiere of The Interview which was due for release on Christmas Day but has been cancelled due to the hacking scandal. Photo: Getty Images

TOKYO - Warning of "serious consequences" if the United States retaliates against it, North Korea insisted on Saturday that it was not behind a damaging cyberattack on Sony Pictures and offered to prove its innocence by proposing a joint investigation with Washington to identify the hackers. 

The message, attributed to an unnamed Foreign Ministry spokesman and carried by North Korea's state-run news service, appears to be the secretive regime's response to President Barack Obama's statement the day before that the United States would retaliate for the hacking, which has shaken one of Hollywood's largest studios.

US officials said the hackers' methods and other clues led them to conclude that North Korea was behind the attack, which resulted in the posting online of Sony's confidential emails and unreleased movies.

The cyberattack and emailed threats of terrorist attacks against movie theatres prompted Sony to cancel the Christmas release of "The Interview," a comedy about the assassination of the North Korean leader, Kim Jong Un.

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North Korea has previously denied responsibility for the hacking, although it called the attacks a "righteous deed" by its "supporters and sympathisers."

On Saturday, North Korea described the US claims that it was behind the attacks as slander. It also warned of serious consequences if the United States rejects its offer of a joint investigation, said the statement carried by the state-run Korean Central News Agency, according to The Associated Press.

It quoted the unidentified spokesman as saying that any joint inquiry would prove that the North was not behind the cyberattack.

"The US should bear in mind that it will face serious consequences in case it rejects our proposal for joint investigation and presses for what it called countermeasures," the spokesman said in the statement, the AP reported.

"We have a way to prove that we have nothing to do with the case without resorting to torture, as what the CIA does," the statement said.

It is unlikely that the Obama administration will take the offer from the North seriously. While some computer experts still express doubts whether the North was actually behind the attack, US officials said it was similar to what was believed to be a North Korean cyberattack last year on South Korean banks and broadcasters.

One key similarity was the fact that the hackers erased data from the computers, something many cyber thieves do not do.

Some US officials have said that North Korea appears to have embraced cyberterrorism as its new weapon of choice for making political points and is possibly trying to extort new concessions out of the United States and its allies.

While North Korea is an impoverished nation with so little Internet usage that it is essentially a black hole in cyberspace, the attacks showed a high level of sophistication and hacking expertise.

The hackers did considerable commercial damage to Sony Pictures, posting embarrassing emails, detailed breakdowns of executive salaries, digital copies of unreleased movies and even the unpublished script for an upcoming James Bond movie.

Sony said the threats against theatres left it no choice but to cancel the December 25 release of "The Interview," in which Seth Rogen and James Franco play television journalists who get a scoop interview with Kim and then find themselves recruited by the CIA to kill him.

On Friday, Obama faulted Sony's decision to withhold the movie, saying that it created a precedent of studios giving into intimidation.