WASHINGTON, April 4 (Reuters) - The United States is considering expanding sales of Lockheed Martin Corp-made F-35 fighter jets to five new nations including Romania, Greece and Poland as European allies bulk up their defenses in the face of a strengthening Russia, a Pentagon official told Congress on Thursday.

In written testimony submitted to the U.S. House of Representatives and seen by Reuters, Vice Admiral Mathias Winter - the head of the Pentagon’s F-35 office - said that “future potential Foreign Military Sales customers include Singapore, Greece, Romania, Spain and Poland.” (Reporting by Mike Stone in Washington Editing by James Dalgleish)