Russia and Turkey are working on a contract for Turkey’s new purchase of Russian S-400 missile systems, Interfax news agency cited a senior official at a Russian military cooperation agency.

“We’re gradually working on this question. Most importantly, both sides are intent on continuing cooperation in this sphere”, said Dmitry Shugaev, head of Russia’s federal service for military-technical cooperation.

The move comes as a response to the United States’ sanctions aimed at the country for buying S-400 batteries this year. The US expelled Turkey from its fifth-generation F-35 fighter jet program.

Russia and Turkey signed in 2017 a $2.5 billion agreement for two batteries of Russia’s most advanced long-range anti-aircraft missile system, S-400.

This is the the first such purchase between Russia and a NATO member. A US official said they will continue to try to convince Turkey to back out.

Turkey, however, has refused to withdraw, saying that the S-400 would not pose any threat to NATO, and pointed out that the US’ refusal to sell Turkey Patriot missiles led it to search for other sellers, and that Russia offered a better deal.