The rumours emerged when Ain't It Cool News claimed that it had received a tip-off about an interview with actor Matthias Schweighofer in the German edition of GQ.
According to the report, Schweighofer - who is allegedly in the running for a part in Thor - said that De Niro and Law have already been cast in unspecified roles. However, the interview does not appear on the website for German GQ or the actor's official website.
De Niro has previously collaborated with director Kenneth Branagh on Frankenstein, while Law worked with him on Sleuth.
Thor was originally created for Marvel by Stan Lee, Larry Lieber and Jack Kirby in 1962. The comics focus on the titular thunder god and his efforts to defend the realm of Asgard from its enemies. The film is slated for release in May 2011.