​North Korea offers US joint investigation of Sony cyberattack

North Korea has proposed a joint investigation with the United States into the hack attack against Sony Pictures, according to the state news agency, KCNA.

The offer comes as the FBI formally accused Pyongyang of the attack on Friday and US President Barack Obama promised to “respond proportionally” to the online breach.

READ MORE: FBI formally accuses North Korea in Sony hack

North Korea says it can prove it has nothing to do with the cyberattack on Sony, the KCNA news release said.

The statement by Pyongyang also warns of "grave consequences" if Washington refuses to cooperate in the investigation of the attack and continues to accuse North Korea.

Sony’s network was hacked in November, with masses of private data, including employees’ emails, being made public.

The attack was followed by threats against movie theaters that planned to show “The Interview,” a comedy in which the US intelligence seeks to kill North Korea's leader Kim Jong-un. Sony eventually refused to release the film.

READ MORE: Obama chides Sony’s decision to nix film, says US must retaliate and pass cyber act

Although North Korea has from the start denied being involved in the hacking and the threats, it still praised the cyberattack as a "righteous deed."

A source in the White House recently told the Wall Street Journal that the US was considering redesignating North Korea as a state sponsor of terror. The country had spent 20 years on the terror black list before being taken off it in 2008 by George W. Bush.

Nuclear boost to counter ‘US hostile policy’

It’s not only the allegations of masterminding the Sony hacking that are vexing Pyongyang. The recent UN General Assembly vote in favour of referring North Korea to the International Criminal Court over alleged crimes against humanity has been seen by the hermit state as an invasion attempt by the US.

North Korea warned on Saturday it would respond by boosting its nuclear power capability.

"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under the guise of human rights has become apparent, the idea of de-nuclearizing the Korean peninsula itself is no longer valid," the North's foreign ministry spokesman said on KCNA, as cited by Reuters.

"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.

North Korea has so far carried out three nuclear tests, the latest one nearly two years ago. Pyongyang has claimed the nuclear activity is purely for self-defense.