File Photo: Military vehicles with S-400 air defense system drive past the Red Square during Victory Day parade rehearsal in Moscow, Russia, May 6, 2010. (Xinhua/Lu Jinbo)

WASHINGTON, April 3 (Xinhua) -- U.S. Vice President Mike Pence on Wednesday warned Turkey against purchasing the S-400 air defense system from Russia, despite Ankara's firm stance on the deal.

"Turkey must choose," Pence said at an event in Washington to celebrate the 70th anniversary of the North Atlantic Treaty Organization (NATO).

"Does it want to remain a critical partner in the most successful military alliance in history of the world? Or does it want to risk the security of that partnership by making reckless decisions that undermine our alliance," the vice president added.

Pence's tough remarks came after Turkey insisted on Wednesday that the Russian deal was done, indicating a serious friction between the two NATO allies.

"The S-400 deal is a done deal and we will not step back from this," Turkish Foreign Minister Mevlut Cavusoglu said at the same event.

"We don't see our relations with Russia as an alternative to our relations with others. And nobody, neither the West nor Russia, can ask us to choose between," Cavusoglu said.

U.S. Defense Department said on Monday that it had halted delivery of F-35 related parts to Turkey because of the latter's purchase of the S-400 defense system from Russia.

In December 2017, Turkey finalized a deal with Russia over the purchase of the S-400 surface-to-air missile system. Russia will deliver the first batch of the defense batteries in late 2019.

Meanwhile, Turkey plans to purchase 100 F-35 fighter jets from the United States, and its pilots are training on the weapons platform at Luke Air Force Base in Arizona of the United States.