- Turkish President Tayyip Erdogan this week flagged potential further cooperation with Russia on defence industry projects including fighter jets and submarines even as the United States warned it could respond with more sanctions.
Turkey received the first deliveries of the S-400 surface-to-air systems in July 2019, prompting Washington to begin removing the NATO ally from its F-35 stealth fighter programme over security concerns.
The following timeline presents the main developments in the programme and Ankara's relations with the United States.
Dec. 29, 2017 - Turkey and Russia sign an accord on deliveries https://reut.rs/3kWmqCW of Russian S-400 surface-to-air missile batteries, reportedly worth around $2.5 billion.
June 19, 2018 - A U.S. Senate committee passes a spending bill https://bit.ly/2Y8fxWN that includes a provision to block Turkey's purchase of Lockheed Martin F-35 fighter jets unless it drops the plan to buy the S-400s.
March 28, 2019 - U.S. Senators introduce a bipartisan bill https://reut.rs/39V5MNR to prohibit the transfer of F-35s to Turkey unless the U.S. administration certifies that Ankara will not take delivery of the S-400s.
June 7, 2019 - The United States decides https://bit.ly/3im4WOA to stop accepting any additional Turkish pilots to train on F-35 fighter jets.
July 17, 2019 - The United States says it was removing https://reut.rs/3B29qkL Turkey from the F-35 programme; Ellen Lord, Undersecretary of Defense for acquisition and sustainment, says Turkey would no longer receive more than $9 billion in projected work.
July 25, 2019 - Russia completes https://reut.rs/3FiZdTQ the first shipment of its S-400 systems to Turkey, according to Turkish military officials.
Sept. 15, 2019 - Turkey's defence ministry confirms https://reut.rs/3B1OQkL delivery of a second battery of S-400s.
Nov. 12, 2020 - Turkey's Defence Minister Hulusi Akar says Turkey is ready to discuss U.S. concerns about the technical compatibility of Russian S-400 defence systems and U.S.-made F-35 jets, renewing Ankara's call for a joint working group with Washington on the issue.
March 24, 2021 - U.S. Secretary of State Antony Blinken, in a meeting with Turkish Foreign Minister Mevlut Cavusoglu, urges Ankara to drop the S-400 system. In the same meeting, Cavusoglu told his U.S. counterpart that its purchase was "a done deal."
July 21, 2021 - U.S. President Joe Biden is committed to maintaining sanctions on Turkey under the Countering America's Adversaries Through Sanctions Act (CAATSA) for buying Russian missile defences and would impose further sanctions if Ankara bought further major arms systems from Moscow, according to a senior U.S. diplomat.
Aug. 23, 2021 - The Interfax new agency reports the head of Russia's arms exporter as saying Russia and Turkey were close to signing a new contract to supply Ankara with more S-400s in the near future.
Sept. 26, 2021 - Turkish President Tayyip Erdogan says Turkey still intends to buy a second batch of missile defence systems from Russia.
Sept. 30, 2021 - Turkey is considering more joint defence industry programmes with Russia including fighter jets and submarines, President Erdogan says after talks with President Vladimir Putin. Erdogan did not mention further S-400 purchases or U.S. sanctions, but said "Turkey would not back down."