Tensions between the U.S. and North Korea soared over the weekend after Washington officially blamed Pyongyang for the cyberattack on entertainment giant Sony Pictures on Friday, the day after a U.N. resolution on its woeful human rights record was adopted.

Warning of “grave consequences” for what it calls groundless accusations, the North proposed a joint inquiry into the Nov. 24 hacking incident on Saturday. The U.S., in turn, dismissed the proposal, saying it stood by the Federal Bureau of Investigation’s conclusion that the North was behind the hacking.

“The government of North Korea has a long history of denying responsibility for destructive and provocative actions,” White House National Security Council spokesperson Mark Stroh said in a press statement. “If the North Korean government wants to help, they can admit their culpability and compensate Sony for the damages this attack caused.”

On Friday, President Barack Obama pledged to respond “proportionally” to the North’s alleged attack on Sony that forced the firm to cancel the Christmas release of “The Interview,” a comedy about two journalists on a mission to assassinate the North’s leader Kim Jong-un.

“We will respond. We will respond proportionally and we will respond in a place and time and manner that we choose,” Obama said during his year-end press conference.

“We cannot have a society in which some dictator someplace can start imposing censorship here in the U.S. If somebody is able to intimidate folks out of releasing a satirical movie, imagine what they start doing when they see a documentary they don’t like, or news reports they don’t like.”

He did not specify what the “proportional response” would be. But one option could be relisting the North as part of state sponsors of terrorism, analysts said.

Asked whether Washington is considering putting the North back on the terrorism sponsor list, U.S. State Department spokesperson Jen Psaki said that the U.S. has “the right to use all necessary means.”

“As the president outlined in his press conference, we, of course, reserve the right to use all necessary means ― diplomatic, informational, military and economic as appropriate and as consistent with domestic and international law ― in order to protect and defend our nation, our allies and our interests,” she said during a press briefing.

In 2008, the Bush administration removed the North from the terrorism list in a move to spur international efforts to denuclearize the country. Pyongyang was put on the list in 1988 after a North Korean spy bombed a South Korean passenger plane in November 1987, killing 115 people.

The listing would impose economic sanctions on the North. Given the level of the North’s isolation, it would not have much of an impact on the already debilitated North Korean economy. But it would further damage its national image and ties with the outside world, observers said.

On Friday, the FBI, the U.S. investigation authorities, announced that it has “enough information” to conclude that the North is responsible for the November cyberattack on Sony Pictures Entertainment.

“We are deeply concerned about the destructive nature of this attack on a private sector entity and the ordinary citizens who worked there. Further, North Korea’s attack on SPE reaffirms that cyberthreats pose one of the gravest national security dangers to the U.S.,” it said in a statement.

“North Korea’s actions were intended to inflict significant harm on a U.S. business and suppress the right of American citizens to express themselves. Such acts of intimidation fall outside the bounds of acceptable state behavior.”

Explaining the North’s culpability, the FBI stated that its technical analysis of the data deletion malware used in the attack revealed links to other malware that the North had previously developed. It also found “significant overlap” between the infrastructure used in the attack and other malicious cyberactivity the U.S. previously linked directly to the North.

The authorities also said that the tools employed in the attack on Sony had similarities to the North’s 2013 cyberattack against South Korean banks and media outlets.

The North’s unidentified Foreign Ministry spokesperson denied responsibility for the cyberattack, calling for a joint probe with the U.S. into the incident.

“Anyone, if he or she wants to level criminal charges against a sovereign state, he or she should put forward clear evidence,” the spokesperson said. “We have means to establish our innocence without torturing anybody ― the torture the FBI does (as an interrogation method).

“The U.S. should bear in mind that it would trigger a grave consequence should it reject our proposal for a joint probe and continue to talk about countermeasures.”

Following the FBI announcement of the North’s responsibility, Seoul condemned Pyongyang and vowed to strengthen cybersecurity cooperation with Washington.

“The government, with its position that such cyberattacks and threats should never be tolerated under any circumstances, will seek to ensure safety in cyberspace through international cooperation, including sharing information on the cyberattacks on Sony Pictures with the U.S. government and strengthening the deterrence and response posture against similar cyberattacks and threats,” Seoul’s Foreign Ministry said in a press release.

Meanwhile, the North’s Foreign Ministry said Saturday that it “fully rejects” the human rights resolution that the U.N. General Assembly adopted last Thursday. Berating the U.S. for supporting the resolution, it also said that the Sept. 19, 2005, joint statement, under which it committed itself to dismantling its nuclear arms, has been nullified.

“Under circumstances where the U.S. hostile policy, intended to militarily invade our republic on the pretext of human rights, has become clear, the denuclearization of the Korean Peninsula … this expression itself does not make sense anymore,” the ministry said.

“We will spur our efforts to strengthen our self-defense capabilities including nuclear capabilities.”

Seoul expressed regret over the statement, urging Pyongyang to take “concrete, substantive” steps to improve the North’s human rights conditions in line with the U.N. General Assembly’s resolution.

By Song Sang-ho (sshluck@heraldcorp.com)