Moves by the US Senate and members of the House of Representatives to block the delivery of the Lockheed Martin F-35 to Turkey have intensified, increasing tensions with Ankara.

The US Senate passed the 2019 National Defense Authorisation Act on 18 June with a clause that would stop the delivery of the F-35 to Turkey.

Passage of the authorisation bill came three days after a bi-partisan group of US representatives wrote a letter to Secretary of Defense James Mattis, asking him to block the F-35 deliveries.

Lockheed is scheduled to roll-out the first completed F-35A ordered by the Turkish air force on 21 June.

“Our concern about the sale of F-35 jets to Turkey comes against the backdrop of Turkey's planned purchase of the Russian S-400 surface-to-air missile system,” the US Representatives wrote in their letter. “As a NATO ally, Turkey integrates its military systems with those of other NATO members. The United States and NATO have warned Turkey that the integration of the Russian S-400 missile system with the F-35 aircraft would threaten exposure of our most closely guarded military secrets to a major power hostile to NATO and US interests.”

The letter also highlighted a number of other concerning events including Turkish military attacks on US partners in Syria, President Tayyip Erdoğan's security forces attacking peaceful protesters assembled in Washington, DC, hostile statements towards US ally Israel and arbitrary arrests of tens of thousands in Turkey.

President Erdoğan seemed to up the ante again on 14 June when he reportedly announced in an interview on Turkish 24 TV that he had reached out to Russian President Vladimir Putin with a proposal for Turkey to jointly produce the S-500 anti-aircraft missile system with Russia.

The efforts by US lawmakers come as Lockheed Martin plans to formally deliver an F-35A Lighting II to Turkey in a ceremony in Fort Worth, Texas on 21 June. The F-35A will remain in the USA until Turkish pilots are trained to operate the aircraft, upon which time it will be flown to Turkey.

Turkish Prime Minister Binali Yıldırım criticized the US Senate vote on June 19, according to Hurriyet Daily News, a Turkish publication.

“The decision is an unfortunate development,” he said. “Turkey is not without alternatives. Such attempts are regrettable and goes against the soul of strategic partnership.”