Turkey has signed a deal with Russia to buy S-400 antiaircraft missile systems in its first major weapons purchase from Moscow.
Turkish newspapers on September 12 quoted Turkish President Recep Tayyip Erdogan as saying that Ankara already paid the deposit on the deal, estimated to be worth up to $2.5 billion.
Speaking to journalists aboard his presidential plane returning from a trip to Kazakhstan, Erdogan also said that he and Russian President Vladimir Putin were "determined on this issue."
Moscow confirmed the accord, with Vladimir Kozhin, a military adviser to Putin, saying that "the contract has been signed and is being prepared for implementation."
Kozhin also said that "all the decisions made for the contract strictly comply" with Russia's strategic interests.
The accord, Ankara's most significant weapons purchase from a non-NATO supplier, has raised concerns in the West over technical compatibility with NATO equipment.
An unidentified NATO official was quoted as saying that the alliance did receive information on the purchase of the Russian-made missile system by Turkey.
The official also said that it was "up to allies to decide what military equipment they buy," adding, "What matters for NATO is that the equipment allies acquire is able to operate together."
"No NATO ally currently operates the S-400," the official added.
Joseph Dunford, the U.S. chairman of the Joint Chiefs of Staff, said in July that the purchase of the S-400 air-defense system from Russia "would be a concern" for Washington.
The Pentagon has also sounded the alarm, saying that "generally it's a good idea" for NATO allies to buy interoperable equipment.
But Erdogan insisted on September 12 that Turkey was free to make military acquisitions based on its defense needs.
"We make the decisions about our own independence ourselves," he said. "We are obliged to take safety and security measures in order to defend our country."
Russia says the S-400 surface-to-air missile system has a range of 400 kilometers and can shoot down up to 80 targets simultaneously.
It deployed the system at its air base near Latakia in western Syria in December 2015 after a Turkish fighter jet shot down a Russian warplane on the Turkish-Syrian border in late 2015, causing a diplomatic rift between Moscow and Ankara.
But Turkey has been establishing closer links with Russia after its recent souring of ties with the United States over a number of issues, including U.S. support of Syrian Kurdish fighters that Ankara considers terrorists.
Meanwhile, Russia's relations with the United States and NATO have been in crisis over its annexation of Ukraine's Crimea region in March 2014, its backing of separatists in eastern Ukraine, and other issues.