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

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.

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”.

“When The Interview trailer came out the North Korean ambassador to the UN made representations to the UN saying that here would be a response,” he said.

“It is only a few months ago that a hairdresser in Ealing was offering a Kim Jong-Un haircut and two senior diplomats from the North Korean embassy went there and told him he couldn’t do that because it was very disrespectful.”

He said it was a “plausible possibility” as Pyongyang had carried out previous hacking attacks, but he warned there is a lot of reporting about North Korea which is not correct because of the problems getting news out of the closed country.

Some reports suggest North Korea may have enlisted Chinese hackers to carry out the attack.

Last year, Sky News reported US security analysts had identified an anonymous-looking tower block in Shanghai as the possible headquarters of a global hacking operation allegedly run by the Chinese military – the People’s Liberation Army Unit 61398.

In May, five members of China’s People’s Liberation Army were charged with hacking US companies for trade secrets.

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

“Sony Pictures continues to work through issues related to what was clearly a cyber attack last week,” a company spokesperson said in a statement.

“The company has restored a number of important services to ensure ongoing business continuity and is working closely with law enforcement officials to investigate the matter.”

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.”