The Department of Defense has halted delivery of Lockheed Martin F-35 Lightning II parts and manuals to Turkey after Ankara refused to cancel an order for a Russian-built radar.

The US and its allies are concerned that Turkey’s plan to buy the Almaz-Antey S-400 Triumf surface-to-air missile system could expose vulnerabilities of the stealth aircraft – weaknesses which could then be exploited by Russia. The radar 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).

"The United States has been clear that Turkey's acquisition of the S-400 is unacceptable. Therefore, the DoD has initiated steps necessary to ensure prudent programme planning and resiliency of the F-35 supply chain. Secondary sources of supply for Turkish-produced parts are now in development," says the Pentagon. "We very much regret the current situation facing our F-35 partnership with Turkey, and the DoD is taking prudent steps to protect the shared investments made in our critical technology. Our important dialogue on this matter will continue, however, until they forgo delivery of the S-400, the United States has suspended deliveries and activities associated with the stand-up of Turkey's F-35 operational capability. Should Turkey procure the S-400, their continued participation in the F-35 program is at risk."

Lockheed Martin officially presented the first F-35A fighters to Turkey in a June 2018 rollout ceremony in Fort Worth, Texas. However, Turkey was not expected to receive the stealth fighter in its own airspace until 2020. In the meanwhile, its pilots had been training on the new aircraft at Luke AFB, Arizona and its aircraft maintainers have begun training at Eglin AFB, Florida.

To entice Turkey into giving up the S-400, the USA has instead offered Raytheon’s Patriot missile system. The country has refused the trade, however.

Removing Turkey from the F-35 programme would cause supply chain disruption. In coordination 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.

Previously, the Pentagon was opposed to removing Turkey from the F-35 programme because of how integrated the country was in the aircraft’s supply chain. However, as the country has refused to give up the anti-aircraft system, the Defense Department has moved gradually toward cutting off deliveries.

Turkey had planned to purchase 100 F-35As. A total of 30 are scheduled for delivery by the end of 2022.

This article was updated on 2 April with a statement from the Pentagon