North Korea has said US accusations that it was involved in a cyber-attack on Sony Pictures were "groundless slander" and that it wants a joint investigation into the incident with the United States.

An unnamed spokesman of the North's foreign ministry said there would be "grave consequences" if Washington refused to agree to the joint probe and continued to accuse Pyongyang.

On Friday, President Barack Obama blamed North Korea for the devastating cyber-attack, which led to the Hollywood studio ­cancelling the release of The Interview, a comedy about the fictional assassination of North Korean leader Kim Jong-un.

In its first substantive response to the accusation, the isolated North Korea said it could prove it had nothing to do with the massive hacking attack.

"We propose to conduct a joint investigation with the US in response to groundless slander being perpetrated by the US by mobilising public opinion," the North Korean spokesman said.

"If the US refuses to accept our proposal for a joint investigation and continues to talk about some kind of response by dragging us into the case, it must remember there will be grave consequences," the spokesman said.

Earlier, the FBI announced that it had determined North Korea was behind the hacking of Sony, saying Pyongyang's actions fell "outside the bounds of acceptable state behaviour".

Obama said North Korea appeared to have acted alone. ­Washington began consultations with Japan, China, South Korea and Russia, seeking their assistance in reining in North Korea.

Japan and South Korea said they would co-operate. China, North Korea's only major ally, has yet to respond, but a Beijing-run newspaper said The Interview was not a movie for Hollywood and US society to be proud of.

"The vicious mocking of Kim is only a result of senseless cultural arrogance," the newspaper said.

It was the first time the United States had directly accused another country of a cyber-attack of such magnitude on American soil and set up a possible new confrontation between longtime foes Washington and Pyongyang.

Obama said he wished Sony had spoken to him first before pulling the release of the film, suggesting it could set a bad precedent. "I think they made a mistake," he said.

Michael Lynton, chief executive of Sony Pictures Entertainment, insisted the company did not ­capitulate to hackers and said it was still looking for alternative platforms to release The Interview. A Sony spokeswoman had earlier said the company did not have further release plans for the $44 million film starring Seth Rogen and James Franco.

Despite Obama's stern ­warning to North Korea, his options for responding to the computer attack appeared limited. The president declined to be specific about any actions under consideration.

North Korea has been subject to US sanctions for more than 50 years, but they have had little effect on its human rights policies or its development of nuclear weapons.

The FBI gave scant details on how it concluded that North Korea was behind the attack.