Moscow – Moscow and Ankara confirmed on Tuesday that they signed a deal in which Moscow would sell Ankara S-400 missiles.

“Signatures have been made for the purchase of S-400s from Russia. A deposit has also been paid as far as I know,” President Recep Tayyip Erdogan said on Monday.

He stated that he and Russian President Vladimir Putin “are determined on this issue.”

This is the largest deal that Turkey signs with a country from outside NATO. It represents an advanced political step in the journey of normalizing relations between the two sides after a crisis that erupted when a Turkish jet shot down a Russian aircraft over Syria in 2015.

Turkey’s announcement of its intention to purchase the Russian missile system triggered however negative reaction from Ankara’s allies in NATO.

US Defense Secretary James Mattis stated earlier that the US will not impede the S-400 missiles deal, saying that it is a Turkish “sovereign decision”. He did complain however over the incompatibility between the Russian system and the NATO systems.

Russian analysts also voiced their skepticism.

Center for Strategic Studies Director Ivan Konovalov declared that the prospect of signing the deal “is part of the political game,” but he added that nothing “can be ruled out at this current time.”

Russia says that S-400 missiles are designed to protect vital political, economic, administrative and military facilities from airstrikes, and are considered of the most advanced systems in the air- and anti-missiles defense.

China was the first country to purchase this system from Russia.