The US Federal Bureau of Investigation yesterday said North Korea was behind a cyber-attack on Sony Pictures over a film about its leader Kim Jong-un.

The agency said analysis of malware showed links to North Korea. Earlier the White House labelled the Sony breach a serious national security matter.

Sony withdrew The Interview, a new comedy film about North Korea's leader, after threats from hackers. Hackers have already released sensitive information stored on Sony computers.

They later issued a warning to members of the public planning to see The Interview. Referring to the 11 September 2001 terror attacks, they said "the world will be full of fear" if the film was screened.

Many cinemas scrapped plans to show The Interview, and Sony then cancelled the release of the film altogether - moves criticised in Hollywood as an attack on the freedom of expression.

The Interview, made by Sony Pictures, features James Franco and Seth Rogen as two journalists who are granted an audience with North Korean leader Kim Jong-un. The CIA then enlists the pair to assassinate him. The film was due to have been released over Christmas.