A US senator has prepared a proposal to buy Russian-made S-400 air defense systems from Turkey in a bid to overcome the impasse between Washington and Ankara over Turkey’s participation in a program to produce F-35 Lightning II fifth-generation fighter jets, Defense News reported.

According to Defense News, Senate Majority Whip John Thune has proposed an amendment to the 2021 National Defense Authorization Act (NDAA) that would allow the purchase to be made using the US Army’s missile procurement account.

Meanwhile, Senate Foreign Relations Committee Chairman Jim Risch has introduced a tougher amendment, which envisages slapping Turkey with sanctions under CAATSA (Countering America's Adversaries Through Sanctions Act) 30 days after NDAA enters force.

According to the paper, although Thune and Risch are both influential senators, "there’s no guarantee either of their amendments would receive consideration to be included in the massive NDAA · or, if passed into the Senate bill, that they would survive negotiations with the House."

Russia announced in September 2017 that it had signed a $2.5 billion deal with Turkey on the delivery of S-400 anti-aircraft missile systems to Ankara. Under the contract, Ankara received a regiment set of S-400 air defense missile systems (two battalions). The deal also envisages partial transfer of production technology to the Turkish side.

Turkey is the first NATO member state to purchase such air defense missile systems from Russia. The deliveries of S-400 launchers to Turkey began on July 12, 2019.

Turkey’s decision to acquire the Russian-made S-400 surface-to-air missile systems has caused a sharply negative reaction from the United States and NATO as a whole. The United States is not abandoning its attempts to make Turkey give up the Russian air defense systems.

As Turkey has not yielded to the pressure and has said that it will not give up the S-400 systems, Washington has excluded Ankara from the US program of developing the fifth-generation F-35 fighter-bomber.

The United States is also threatening Turkey with unilateral sanctions over the purchase of S-400 air defense systems but is in no hurry to take these steps out of fear of further worsening relations with a major NATO ally while Ankara has warned it will not leave the imposition of these restrictions unanswered.

The S-400 ‘Triumf’ is the most advanced long-range air defense missile system that went into service in Russia in 2007. It is designed to destroy aircraft, cruise and ballistic missiles, including medium-range weapons, and can also be used against ground installations. The S-400 can engage targets at a distance of 400 km and at an altitude of up to 30 km.