In a letter sent to US Representative Mac Thornberry, chairman of the Armed Services Committee, US Secretary of Defense James Mattis acknowledged concerns about the Turkish government, while opposing US lawmakers’ efforts to remove the country from the F-35 Lightning II programme, saying the loss of the nation from the supply chain would delay delivery of some aircraft for up to two years.

The top objection from lawmakers to Turkey receiving the Lockheed Martin F-35 stealth fighter is the nation’s agreement with Russia to buy the Almaz-Antey S-400 Triumf system. That surface-to-air missile system is considered one of the most advanced on the export market and is advertised by Rosoboronexport as having an "anti-stealth range" of up to 81nm (150km).

Lawmakers also complain about what they say is Turkish President Recep Tayyip Erdoğan’s decreasing respect for the rule of law, imprisonment of an American pastor, diminishment of individual freedoms, consolidation of power and strategic military decisions that are out of line with US interests.

In response, a bi-partisan group of US Representatives sent a letter to Mattis on 15 June, asking him to block the F-35 deliveries. The US Senate passed the 2019 National Defense Authorisation Act on 18 June with a clause that would also block the aircraft delivery.

Until now, the Defense Department has been silent on lawmaker opposition to Turkey’s involvement in the F-35 programme.

“Removing Turkey could trigger a supply chain disruption for the US military and our partners, as well as increase other program costs,” Mattis says in his letter to the House Armed Services Committee Chairman on 7 July. “If the Turkish supply chain was disrupted today, it would result in an aircraft production break delaying delivery of 50-75 F-35s, and would take approximately 18-24 months to re-source parts and recover.”

In co-ordination with Northrop Grumman, the main fuselage manufacturer for the F-35, Turkish Aerospace Industries manufactures and assembles centre fuselages, produces composite skins and weapon bay doors, and fibre placement composite air inlet ducts. In total, ten different Turkish firms make parts for every F-35 manufactured.

Turkey plans to purchase 100 F-35As, with its first batch of 14 already contracted. A total of 30 are scheduled for delivery by the end of 2022.

“I understand and agree with Congressional concerns about the authoritarian drift in Turkey and its impact on human rights and rule of law including the detainment of American citizens such as Pastor Brunson,” Mattis says in his letter. “The Administration is pressing Turkey on these issues as well as the potential acquisition of the S-400 air defense system.”

The US Department of State is in talks with Ankara to sell the nation the Raytheon MIM-104 Patriot, instead of the Russian S-400, it was revealed this week at the Farnborough air show by US acting assistant secretary for political-military affairs Tina Kaidanow.

The House Armed Services Committee decline to comment on the letter from Mattis.