Turkey conducted military tests using a Russian air defense system and an American-made fighter jet on Nov. 25, 2019, in a move US officials described as "concerning."
Turkish F-16 jets flew over the capital of Ankara as part of a test of the S-400 missile defense system, which the Turkish President Recep Tayyip Erdoğan purchased from Russia for $2.5 billion.
The purchase scuttled plans for Turkey to acquire the latest-generation F-35 Lightning II jet from the US, due to concerns that the Russian anti-aircraft system could exploit the US's most advanced stealth technology. The purchase effectively nixed plans for Turkey to buy the US's Patriot missile system.
One US diplomat said there was a chance that Russia had the ability to access Turkey's S-400 remotely, and use a potential backdoor to observe on NATO allies, according to Defense News.
US lawmakers threatened to mount a campaign to levy sanctions against Turkey after it received delivery of a second battery in August 2019. The 2017 Countering America's Adversaries Through Sanctions Act allows Trump to sanction Turkey for conducting business with Russia.
It remains unclear if Trump will impose sanctions against Turkey, NATO's second-largest military after the US. In 2017, Trump described the sanctions act as "seriously flawed" and said he signed it into law "for the sake of national unity."
"Erdoğan is thumbing his nose at Trump, the US [and] NATO, and crossing another red line on S-400s," Democratic Sen. Chris Van Hollen of Maryland said on Twitter.
Secretary of State Mike Pompeo on Tuesday told reporters that the tests were "concerning," but added he remained optimistic on resolving the impasse.
"We are hopeful. We are still talking to the Turks, still trying to figure out our way through this thing," Pompeo said.
Despite objections on the S-400, President Donald Trump, who met with Erdoğan on Nov. 13, 2019, described his broader conversations with the Turkish president as "wonderful."
This article originally appeared on Business Insider. Follow @BusinessInsider on Twitter.
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