"It's a done deal", Çavuşoğlu said during the "NATO Engages" event in Washington, D.C., to mark the alliance's 70th anniversary.
Turkey had indicated that it would purchase the Russian S-400 missile defense system instead of the U.S. Patriot system - a move that concerned Pentagon leadership because of Turkey's existing partnership on the F-35 fighter jet program.
But Turkish Foreign Minister Mevlut Cavusoglu ruled out a change of heart, saying: "The S-400 deal is a done deal and we will not step back from this", he said.
Ankara has rebuffed persistent warnings from Washington that deploying the S-400 system would compromise Turkey's role in the F-35 program. Russian Federation will deliver the first batch of the defence batteries in late 2019.
This week, Washington announced it was halting deliveries of F-35 fighter jets to Turkey until Ankara walked back on its deal with Moscow.
As a result, US government leaders have repeatedly threatened to shut down Turkey's participation in the F-35 program if the Russian purchase goes through.
Vice President Mike Pence, in wide-ranging remarks on Wednesday before a gathering of North Atlantic Treaty Organisation members in Washington, D.C., outlined a series of reforms the Trump administration has taken to boost NATO's military readiness and position it as a force that can combat Russian aggression, as well as that of regional terror groups.
"That S-400 will be in the midst of a NATO-integrated air missile defense system as well", he said. The United States this week halted delivery of equipment related to the stealthy F-35 fighter aircraft to Turkey.More news: Manhunt over, focus in Nipsey Hussle case shifts to court
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"We will agree on new measures of support for our close partners Georgia and Ukraine", NATO Secretary General Jens Stoltenberg told reporters ahead of Thursday's session.
Turkey has continued to maintain that it has the right under worldwide law to defend against national security threats including any potential danger posed in northern Syria.
Britons were nearly equal in the urge to protect as Americans, but opinion was an "even split" on whether to defend Turkey, while the French were more willing to defend the USA and others apart from the Ukraine, Turkey, and Romania.
Fears for the safety of the fighters has led the United States to delay President Donald Trump´s decision to remove U.S. troops from Syria. Officials have said that Turkey's acquisition of both U.S. and Russian systems could give Moscow access to sophisticated American technology and allow it to find ways to counter the F-35.
"This is a very rocky relationship", says Asli Aydintasbas, an Istanbul-based senior policy fellow at the European Council on Foreign Relations.
Turkey expressed irritation with the State Department Wednesday over its description of a meeting between Secretary of State Mike Pompeo and Turkish Foreign Minister Çavuşoğlu earlier this week. "We have to shoulder responsibility in order to continue safeguarding it, in our own interest", he said.
The state has also carried two-cross border operations in the region.