A hacking attack that crippled key systems at Sony Pictures and led to blockbuster movies being leaked online is increasingly being blamed on Pyongyang after the FBI said some of the software used was compiled in Korean.

The attack, by a group calling itself the "Guardians Of Peace", left the firm's corporate email down for a week and enabled hackers to steal at least four movies, including Second World War drama "Fury" starring Brad Pitt which is yet to be released on DVD.

It has since been downloaded more than one million times.

The Korean malware has led to increased speculation that Sony was targeted in revenge for its movie "The Interview" - a comedy about two journalists recruited by the CIA to assassinate North Korean leader Kim Jong-Un.

The Pyongyang government had previously described the film as "undisguised sponsoring of terrorism" and an "act of war."

The FBI has given US firms advice on spotting and handling the malware in a five-page report, but it did not confirm if it was a response to the incident at Sony.

Cybersecurity experts said it marked the first attack of its kind on a major US company.

Hackers have used malware similar to that described in the FBI report to launch attacks on businesses in South Korea and the Middle East, including one against oil producer Saudi Aramco that knocked out some 30,000 computers.

"I believe the coordinated cyberattack with destructive payloads against a corporation in the US represents a watershed event," Tom Kellermann, chief cybersecurity officer with security software maker Trend Micro Inc, said.

"Geopolitics now serve as harbingers for destructive cyberattacks."