The S-400 is Russia's most advanced long-range anti-aircraft missile system and can carry three types of missiles capable of destroying targets including ballistic and cruise missiles.
Speaking to the Turkish press, President Erdoğan said that Turkey is "obliged to take safety and security measures" for its national defense, recalling that several weapons systems that Ankara sought to purchase were not made available in the west.
Erdogan said in comments reported by Turkish media that accords had been signed and a deposit has been paid for the acquisition of the S-400 surface-to-air missile defence systems.
Ankara's relations with North Atlantic Treaty Organisation ally Germany, hit by a deepening row, are expected to improve alongside a general upturn in ties with the European Union early next year, boosting Turkish export and tourism prospects, an adviser to President Tayyip Erdogan told Reuters. "We are taking precautions and we will continue to do so", Erdoğan said on September 13 in a speech to ruling Justice and Development Party (AKP) mayors in Ankara. He said Sunday that a down payment has already been transferred to Moscow and he and Russian President Vladimir Putin were determined that the agreement should proceed. "So what? Are we going to wait for you?"
A military adviser to Russian President, Vladimir Putin, Vladimir Kozhin, said the S-400 contract with Turkey was "strictly compatible with our strategic interests".
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". Turkey commands the second largest standing military inside of 29-member-state-NATO.
Last month, the Pentagon expressed concern about Turkey's intention to purchase the system.
Turkey's relations with Washington have deteriorated due to the cooperation between Washington and the Kurdish People's Protection Units (YPG) in Syria.