North Korean leader Kim Jong Un stands on the conning tower of a submarine during his inspection of the Korean People's Army Naval Unit 167 in this undated photo released June 16, 2014. REUTERS/KCNA

SEOUL (Reuters) - North Korea proposed a joint investigation with the United States on the cyber attack against Sony Pictures, calling the charge by the FBI that it was behind the attack "slander", state media said on Saturday.

An unnamed spokesman of the North's foreign ministry said there would be "grave consequences" if Washington refused to agree to the joint probe and continued to accuse the North, the official KCNA news agency reported.

(Reporting by Jack Kim; Editing by Jeremy Laurence)