The FBI has warned US firms to guard against malicious software following an attack on Sony Pictures Entertainment that crippled key systems and led to unreleased films being posted on pirate websites.

The Reuters news agency reported that while the five-page FBI report it had seen offered advice on spotting and handling the malware, it did not confirm if it was a response to the incident at Sony.

The attack left its corporate email down for a week and enabled hackers to steal at least four movies, including Second World War drama "Fury" starring Brad Pitt.

It has since been downloaded more than one million times.

The FBI said some of the software used by the hackers was compiled in Korean, intensifying 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."

Sky's Asia Correspondent Mark Stone, in Beijing, said that while North Korea is yet to comment on the cyber attack on Sony, it "takes any slight on its leader very, very seriously."

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.

(Sky news)