North Korea has proposed a joint investigation with the US into the hacking attack against Sony Pictures Entertainment, warning of "serious" consequences if Washington does not cooperate.

The call for a new investigation came a day after the Federal Bureau of Investigation said it had evidence Pyongyang was responsible for the attack, prompting President Barack Obama to declare that the United States would respond "proportionately" to the cyberattack.

On Saturday, an unidentified Foreign Ministry spokesman in the capital Pyongyang said North Korea was able to prove it was not responsible for the hacking, and urged the United States must accept its proposal for the joint investigation.

"The U.S. should bear in mind that it will face serious consequences in case it rejects our proposal for [a] joint investigation and presses for what it called countermeasures while finding fault with'' North Korea, the spokesman said in a statement carried by Pyongyang's official Korean Central News Agency.

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

But the US says it stands by assertion that North Korea is behind the cyberattack.

"The Government of North Korea has a long history of denying responsibility for destructive and provocative actions," Mark Stroh, spokesman for the White House National Security Council, said.

"If the North Korean government wants to help, they can admit their culpability and compensate Sony for the damages this attack caused," Stroh told Reuters news agency.

US officials blame North Korea for the hacking, citing the tools used in the Sony attack and previous hacks linked to the North, and have vowed a response.

"We will respond. We will respond proportionately and we'll respond in a place and time and manner that we choose," Obama told an end-of-year news conference at the White House on Friday.

Insufficient evidence 

Al Jazeera's Shihab Rattansi, reporting from Washington, said that many cyber security experts in the US and around the world had said that the evidence presented by the FBI was "pretty flimsy".

"Malware that may or may not have been used by North Korea in the past does not necessarily prove anything because malware is freely traded amongst hackers," he said, referring to the hostile software the FBI said revealed links to the kind of sofware developed by North Koreans.

"According to Wired magazine, the software used to wipe Sony's servers is freely commercially available and has been used in hacking attacks on Saudi Arabia's petroleum company. That was not blamed on North Korea." 

Last month's break-in resulted in the disclosure of tens of thousands of confidential Sony emails and business files, and escalated to terrorist threats that caused Sony to cancel the Christmas release of the movie "The Interview". The comedy is about a plot to assassinate North Korean leader Kim Jong Un.

Obama declared that Sony "made a mistake" in shelving the satirical film and pledged the US would respond "in a place and manner and time that we choose".

North Korea said on Friday it had nothing to do with the devastating attack, according to its UN diplomat.

"DPRK [Democratic People's Republic of Korea] is not part of this," the diplomat, cited by the Reuters news agency and speaking on condition of anonymity, said on Friday. He declined to comment further.

Source: Al Jazeera and agencies