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 where the Pentagon acts as an intermediary between the defense contractor and a foreign government.

Other U.S. allies have been eyeing a purchase of the stealthy jet including Finland, Switzerland and the United Arab Emirates.

Winter’s full written testimony, which will be made public as soon as Friday, said the United States would respond to all official requests for information about the jet.

Last year, Belgium was the first new customer for the F-35 in years, choosing it over the Eurofighter Typhoon to replace its aging F-16s in a 4 billion euro ($4.55 billion) deal.

Under President Donald Trump the United States has rolled out a “Buy American” plan that relaxed restrictions on sales and encouraged U.S. officials to take a bigger role in increasing business overseas for the U.S. weapons industry.

Lockheed, the jet’s prime contractor, is developing and building three models of the new warplanes for the U.S. military and 10 other countries that have signed up to buy the jets: Britain, Australia, Italy, Turkey, Norway, the Netherlands, Israel, Japan, South Korea and Belgium.

U.S. arms sales to foreign governments rose 13 percent to $192.3 billion in the year ending Sept. 30, the State Department said in November.

Lockheed delivered 91 F-35 fighter jets to the United States and its allies in calendar 2018.