U.S. Considers 'Proportionate' Response to Sony Hacking Attack The attack was an example of 'destructive activity with malicious intent that was initiated by a sophisticated actor,' a White House spokesperson said.

The United States said on Thursday a cyber attack on Sony Pictures blamed on North Korea was a serious national security matter and the Obama administration was considering a proportional response.

White House spokesman Josh Earnest told reporters the attack was an example of "destructive activity with malicious intent that was initiated by a sophisticated actor."

Earnest said he was not in a position to say that North Korea was responsible, but the investigation was "progressing."

U.S. government sources said on Wednesday that U.S. investigators had determined that the attack was "state sponsored" and that North Korea was the government involved.

Earnest said U.S. national security leaders considering the attack "would be mindful of the fact that we need a proportional response." They were also aware that people carrying out such attacks are "often seeking to provoke a response..."

"They may believe that a response from us in one fashion or another would be advantageous to them" by enhancing their standing either among their cohorts or on the international stage, Earnest said.

Hackers who said they were incensed by a film on the fictional assassination of North Korea's leader attacked Sony Corp last month, leaking documents that drew global headlines and distributing unreleased films on the Internet.

North Korea has denied it was behind the Sony hacking, but security experts in Washington said it was an open secret Pyongyang was responsible. The hacking and cancellation of "The Interview" movie's Dec. 25 release appeared to be an unprecedented victory for Pyongyang and its abilities to wage cyber warfare.

In New York on Thursday, a senior North Korean diplomat at the United Nations declined to comment on accusations that Pyongyang was responsible. He also declined to comment on the film's cancellation.

Sony, in its announcement Wednesday on the $44 million raunchy comedy, cited decisions by several theater chains to hold off showing the film. The hacker group that broke into Sony's computer systems had threatened attacks on theaters that planned to show it.

U.S. experts say options for the Obama administration could include cyber retaliation and financial sanctions but the effect of any response could be limited given North Korea's isolation.

Political analysts, including Joel Wit of the 38 North Korea project at Johns Hopkins University in Washington, questioned how easy it would be to enforce sanctions and to ensure the support of China, which is North Korea's biggest economic partner, its neighbor and long-time ally.

The United States has a deep economic relationship with China but is sharply at odds with Beijing over Washington's allegations of cyber spying by Chinese state units on U.S. concerns.

The Republican chairman of the House of Representatives Foreign Affairs Committee, Ed Royce, said the United States should impose new penalties on the already heavily sanctioned North Korea that would "wall off" the country from the international banking system.

The Federal Bureau of Investigation warned theaters and other businesses associated with "The Interview" on Tuesday that they could be targeted in cyber attacks, according to a copy of the document reviewed by Reuters.

Several U.S. national security officials told Reuters the government had no credible evidence, however, of a physical threat to movie theaters.

(Additional reporting by Jim Finkle in Boston, Mark Hosenball and Matt Spetalnick in Washington and Michelle Nichols at the United Nations; Editing by David Storey and Grant McCool)