North Korea says US accusations that it was involved in a cyber attack on Sony Pictures are “groundless slander” and has demanded a joint investigation into the incident.

An unnamed spokesman of the North’s foreign ministry also said there would be “grave consequences” if Washington refused to agree to the joint probe and continued to accuse the North of the attack, the country’s official KCNA news agency reported.

• George Clooney slams decision to pull movie
• Barack Obama vows revenge for cyberattack

“As the United States is spreading groundless allegations and slandering us, we propose a joint investigation with it into this incident,” the spokesman was quoted by KCNA as saying.

“Without resorting to such tortures as were used by the US CIA, we have means to prove that this incident has nothing to do with us.”

The cyber attack prompted Sony Pictures to cancel the Christmas Day release of The Interview, a madcap satire about a fictional CIA plot to kill North Korean leader Kim Jong-un.

On Friday, the FBI said North Korea was to blame for the devastating strike.

US president Barack Obama said North Korea appeared to have acted alone, but Washington has sought help from Japan, China, South Korea and Russia in combating similar attacks.

The government of China, North Korea’s only major ally, has yet to respond to the US call, but a state-run newspaper denounced Sony’s comedy as senseless and arrogant.

Mr Obama said on Friday he believed the studio had erred in cancelling the film’s release.

Sony Pictures said they still hoped to release The Interview on a different platform.

North Korea says to boost nuclear power over hostile US

North Korea has also vowed to boost its “nuclear power” to counter Washington’s hostile policies.

It said it had become apparent the US aimed to invade the North under the guise of human rights abuses.

Member countries of the United Nations on Thursday urged the Security Council to consider referring North Korea to the International Criminal Court (ICC) for crimes against humanity as alleged in a UN inquiry report released earlier this year.

“Now that the US hostile policy with an aim to invade our republic under the guise of human rights has become apparent, the idea of denuclearising the Korean peninsula itself is no longer valid,” the North’s foreign ministry spokesman said on KCNA.

“Our effort to strengthen our defensive military power including nuclear power will be doubly stepped up in every way,” the spokesman said in a statement.

A UN Commission of Inquiry report published in February detailed wide-ranging abuses in North Korea, including prison camps, systematic torture, starvation and killings comparable to Nazi-era atrocities.

It is unlikely to lead to action in the ICC at The Hague, which looks at serious abuses like genocide and other crimes against humanity, because China would likely use its veto power to block it.

North Korea has conducted three nuclear tests and is believed to have materials for up to a dozen atomic weapons but there is no independent assessment that it has mastered the technology to mount a warhead on a missile.