Sony Pictures
The United States spokesperson has refused to comment on whether North Korea was behind the Sony Hacking.Reuters

A hacking attack on Sony Pictures, which forced the cancellation of the release of the film "The Interview" is a very serious national security issue, the United States has said.

According to a White House spokesman, the US believed the hacking was undertaken by a "sophisticated actor" but he refused to comment on the claims that North Korea was behind it.

Sony Pictures announced that it has cancelled the release of "The Interview" on Christmas Day, following three weeks of hacking, leaks and threats. The decision came as officials said that North Korean government was "certainly" behind the hacking attack.

The hacking group – Guardians of Peace -- has been the subject of much controversy after it leaked sensitive data, including salaries and industry emails in the 100 terabytes of data it hacked from the studio.

The group later issued threats of violence against those who attended the movie. It said "the world will be full of fear" if the film was screened, referring o the 11 September 2001 terror attacks.

White House spokesman Josh Earnest said in a briefing on Thursday that US officials had held daily discussion about the Sony cyber attack and were mulling an "appropriate response".

However, he refused to say who exactly was responsible, saying he didn't wish to speculate ahead of an investigation by the Department of Justice as well as the FBI.

Many cinemas scrapped plans to show "The Interview", following which, Sony announced the cancellation of the film altogether, a move which was criticised in Hollywood as an infringement into the freedom of expression.