WASHINGTON (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.”

News of the new customers coincides with U.S. tension with F-35 development partner Turkey over Ankara’s plans to buy a Russian missile defense system.

Foreign military sales like those of the F-35 are considered government-to-government deals