Obama also said the North is believed to have acted alone without working with any other country.

Secretary of State John Kerry also issued a statement condemning the North.

"The United States condemns North Korea for the cyber-attack," he said. "These actions are a brazen attempt by an isolated regime to suppress free speech and stifle the creative expression of artists beyond the borders of its own country."

Senate Foreign Relations Committee Chairman Robert Menendez called for re-designating the North as a state sponsoring terrorism. Pyongyang was put on the U.S. terrorism sponsor list for the 1987 bombing of a Korean Airlines flight, but was removed from the list in 2008 in exchange for progress in denuclearization talks.

North Korea had been suspected of involvement in last month's cyber-attack on Sony from the beginning because the communist nation has expressed strong anger at the controversial movie about a plot to assassinate North Korean leader Kim Jong-un.

Pyongyang has denied involvement, though it lauded the cyber-attack as a "righteous deed."

Earlier, the FBI announced it has determined the North is responsible for the attack.

"As a result of our investigation, and in close collaboration with other U.S. government departments and agencies, the FBI now has enough information to conclude that the North Korean government

is responsible for these actions," the agency said in a statement.

The attack on Sony reaffirmed that cyber-threats "pose one of the gravest national security dangers to the U.S.," the FBI said, adding that the "destructive nature of this attack, coupled with its coercive nature, sets it apart" from other cyber-intrusions.

"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," it said.

The film, "The Interview," tells the story of two American journalists who land an interview with the North Korean leader in Pyongyang but are then recruited by the CIA to kill him. Pyongyang has condemned the movie as the "most undisguised" sponsoring of terrorism.

On Wednesday, Sony ultimately decided to call off the film's release, which had been set for Christmas Day, after hackers threatened attacks on theaters showing the movie, and some large theater chains decided to pull the film from their screens.

Obama said Sony made a mistake in pulling the film.

"We cannot have a society where some dictator someplace can start imposing censorship here in the United States. Because if somebody's going to intimidate them for releasing a satirical movie, imagine what's going to happen when there's a documentary they don't like," Obama said.

"Even worse, if producers and distributors start engaging in self-censorship because they don't want to offend the sensibilities of somebody who frankly probably needs their sensibilities offended ... That's not who we are. That's not who Americans are," he said.

The FBI said that the data deletion malware used in this attack revealed links to other malware that the FBI knows North Korean actors previously developed. There were similarities in specific lines of code, encryption algorithms, data deletion methods, and compromised networks, it said.

The agency also said that it observed a significant overlap between the infrastructure used in the attack and other malicious cyber-activity that has previously been linked directly to North Korea. It also said the tools used in the attack have similarities to a cyber-attack in March of last year against South Korean banks and media outlets.

Homeland Security Secretary Jeh Johnson urged businesses to ensure their cyber-security.

"The cyber attack against Sony Pictures Entertainment was not just an attack against a company and its employees. It was also an attack on our freedom of expression and way of life," Johnson said in a statement, adding that the case underscores the importance of good cyber-security practices.

The case is expected to further dampen hope for resuming negotiations between Washington and Pyongyang. The six-party talks over the North's nuclear program have been stalled since 2008. The

U.S. has demanded Pyongyang demonstrate its denuclearization commitments before resumption of the talks.

Ken Gause, a senior analyst on Korea at CNA Corp., said that how this case would affect the overall U.S.-North Korea relations depends on how the U.S. responds to the attack. But chances of the case having any dramatic impact appear low, he said.

"It really depends on how the U.S. responds. It's all about proportionality. Given that US policy toward N.K. has been benign neglect, there shouldn't be too much impact. If, however, the U.S. was contemplating a positive shift toward NK, as some suggest, this could be a setback," Gause said in comments emailed to Yonhap News Agency.

"Obama up to now has not been too enthusiastic about engaging North Korea unless it is tied to denuclearization, something North Korea is not willing to do. That being the case, I can't see how this dramatically impacts future negotiations," he said. (Yonhap)