The Russian Federal Service for Military Technical Cooperation has denied that U.S. would be able to buy Turkey’s Russian-made S-400 air defense system under legislation proposed in the Senate last week.

“To export military products, a buyer of our weapons must present an end-user declaration to the Russian side. That is why transfer or re-export of such products to third countries is impossible without an official permit from the Russian side,” the press service of Russia’s Federal Service for Military Technical Cooperation told TASS on Tuesday.

As Defense News previously reported, Senate Majority Whip John Thune, R-S.D. has prepared a proposal to buy Russian-made S-400 air defense systems from Turkey. According to the newspaper this is will make it possible 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.

The U.S. routinely buys foreign technology and could both exploit the S-400′s technology and test U.S. tactics, said Jim Townsend, a former Pentagon official for European and NATO policy. If Turkey doesn’t go for the idea, he said, the two countries are still stuck.

“I think the US buying the S-400s from Turkey is a clever way of getting Erdogan out of the jam he put himself in,” Townsend said. “We just want to get the system out of Turkey … and if it enables the Turks to take part in the F-35 then all the better.”

In September 2017, Russia and Turkey have signed a $2.5 billion contract for supplies of new S-400 missile systems. The first batch under the contract was delivered to Ankara by air transport in July 2019.

Currently, the United States is 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.