The FBI is investigating after hackers claiming to work on behalf of Islamic State militants seized control of the Twitter and YouTube sites of the US military.

A senior FBI official has said that the agency has not confirmed widely held suspicions that North Korea is behind the unprecedented cyber attack on Sony’s Hollywood studio.

Sony Pictures Entertainment has refused to comment on the leak of highly sensitive data following a hacking attack, which revealed a female executive reportedly makes almost $1m (€800,000) less than her male counterpart each year.

A growing number of web security experts are casting doubt on the theory that North Korea was behind the hacking attack that crippled Sony Pictures’ computers and possibly leaked unreleased movies online.