ABOARD AIR FORCE ONE, May 14 (Reuters) - President Barack Obama has not expressed a preference for which U.S. agency should be responsible for proposed regulation of derivatives trading, a White House spokesman said on Thursday.

"This is going to cross several different departments and agencies, so we have to work with Treasury and CFTC and others to ensure that we get a proposal that we believe is workable," White House spokesman Robert Gibbs told reporters aboard Air Force One.

On Wednesday, the administration proposed subjecting all over-the-counter derivatives dealers to regulation.

When asked for President Obama's position on which agency should regulate derivatives, he said: "I have not heard him enumerate a preference at this point in time." (Reporting by Jeff Mason; Editing by Gary Hill)