WASHINGTON, Dec. 21  -- President Barack Obama said Sunday that he regards the hacking of Sony Pictures Entertainment (SPE) as an "act of cyber vandalism" rather than an "act of war," while reiterating the U.S. will respond proportionately.

"No, I don't think it was an act of war. I think it was an act of cyber vandalism that was very costly, very expensive. We take it very seriously. We will respond proportionately," Obama told CNN's Candy Crowley, anchor of the Sunday program "State of the Union," in an interview taped Friday.

Obama stood by his criticism of the U.S.-based SPE in its decision to cancel the scheduled Dec. 25 release of its comedy movie "The Interview," which depicts an assassination attempt on Kim Jong Un, leader of the Democratic People's Republic of Korea ( DPRK).

Sony took the step after hackers hacked into its computer system and released some of the stolen data late November. The hackers threatened to disclose more Sony data if it goes ahead with the movie's release, while warning movie-goers to stay away from theaters if they were to screen the movie.

The U.S. Federal Bureau of Investigation (FBI) claimed Friday that investigators had found what it called "enough information" to conclude that the DPRK was "responsible for" the attack.

Speaking at his end-of-the-year news conference Friday, Obama said Sony made a mistake by canceling the release of the movie. He said he wished the company had contacted him directly before taking the action, while vowing to "respond proportionally" to the cyber attack.

The DPRK on Saturday rejected the U.S. accusation that Pyongyang was behind the cyber attack on Sony and proposed conducting a joint investigation with the United States.

"Reference to the past cyber attacks quite irrelevant with the DPRK and a string of presumptive assertions such as 'similarity' and 'repetition' can convince no one," said the official DPRK news agency, KCNA, quoting a DPRK foreign ministry spokesman.

The spokesman also said in a statement that the DPRK will target its retaliation at only those responsible for anti-DPRK acts and their bases, but will not get involved in terrorist attacks on innocent audience in theaters.

It also warned the U.S. of "serious consequences" should it reject the proposal for a joint investigation and insist on taking responsive measures.