Turkish President Recep Tayyip Erdogan has reiterated his country’s resolve to proceed with a purchase of advanced S-400 air defense missile system from Russia, a day after an informal deadline Washington set for Ankara to respond to a rival US offer passed.

AVA- Erdogan was quoted by Turkish broadcaster NTV on Saturday as saying that there was no turning back on the S-400 deal with Russia, and that Turkey was also interested in purchasing US Patriot missiles as long as Ankara's interests were served.
"We have made the S-400 deal with Russia, so it is out of question to take a step back. The job is done. When it comes to the Patriots, we are open to buying them. However, this purchase needs to serve the interests of our country," Erdogan told journalists on the way back from a trilateral summit in the Russian city of Sochi, where he met his Russian and Iranian counterparts to discuss developments in Syria.
"We are continuing our work based on the deal that the S-400s will be delivered in July," the Turkish president added.
Moscow and Ankara reached a final agreement on the delivery of the S-400 missile systems in December 2017.
Turkey, which is a NATO country, has on several occasions stated that it is committed to buying the Russian missile defense system, despite concerns among members of the US-led military alliance that the S-400s cannot be integrated into the NATO air defense system.   
Washington had set an informal deadline of February 15 for Ankara to respond to the rival US offer, warning to withdraw its offer to sell a $3.5 billion Raytheon Co Patriot missile package if Turkey proceeded with the S-400 purchase.
US officials also announced that the deal would jeopardize Turkey’s purchase of Lockheed Martin Corp F-35 fighter jets and possibly result in the United States imposing sanctions.
The S-400, whose full name is the Triumf Mobile Multiple Anti-Aircraft Missile System (AAMS), is an advanced Russian missile system designed to detect, track, and destroy planes, drones, or missiles as far as 402 kilometers away. It has previously been sold only to China and India.
Turkey is striving to boost its air defense, particularly after the US and Germany decided in 2015 to withdraw its Patriot surface-to-air missile system from Turkey's border with Syria, a move that weakened Turkey’s air defense.
Turkey urged the NATO allies at the time to keep those batteries positioned on the border.
Ankara was also angry with the US over its military support for the YPG Kurdish fighters in northern Syria, as well as its failure to extradite US-based opposition cleric Fethullah Gulen, whom Ankara accuses of having masterminded the 2016 coup attempt. Turkey regards the US-baked YPG as a terrorist group affiliated with the outlawed Kurdistan Workers Party (PKK).
Before gravitating towards Russia, the Turkish military reportedly walked out of a $3.4 billion contract for a similar Chinese system. The withdrawal took place under purported pressure from Washington.