The U.S. will take the unprecedented step of imposing sanctions on a NATO ally when Turkey receives a Russian missile defense system, according to a State Department official testifying before Congress.

The delivery of the S-400s will impair military cooperation between the U.S. and Turkey and will damage relations between the countries more generally, Wess Mitchell, U.S. Assistant Secretary of State for European and Eurasian Affairs, said on Tuesday in testimony to the Senate.

The U.S. has been clear that the “acquisition of S-400 will inevitably affect prospects for Turkish military-industrial cooperation with the U.S., including F-35,” Mitchell said, referring to fighter jets that Turkey has ordered from the U.S. “A decision on S-400 will qualitatively change the U.S.-Turkish relationship in a way that would be very difficult to repair.”

The Senate passed a bill last week calling for a freeze of arms sales to Turkey until assessment is made of military and diplomatic ties in light of its potential purchase of the Russian system.

The bill also called for assessment of impacts on other U.S. weapon systems and platforms operated and developed jointly with Turkey, including the F-35 Lightning II Joint Strike aircraft.

Turkey has argued that the S-400s aren’t a threat to allies and it would prefer to meet its needs within the NATO network. President Recep Tayyip Erdogan has complained repeatedly that he was forced to seek alternatives after facing hurdles in obtaining weapons systems from the U.S. and Europe under the conditions Turkey wanted, which included some local production and transfer of technology.

“It won’t be possible for that defense system to categorize allies as foes,” Foreign Minister Mevlut Cavusoglu said June 22, arguing that NATO-member Greece also bought a similar missile defense system from Russia in the past.

The purchase has added to a long list of issues that are straining ties between Turkey and the U.S., which claim the two largest armies in NATO.

Since a failed coup attempt against Erdogan in 2016, Turkey has been pleading with the U.S. to extradite Fethullah Gulen, an elderly cleric living in Pennsylvania who Turkey says was the putsch mastermind.

The U.S., in turn, has been threatening consequences should Turkey not release Americans imprisoned in the aftermath of the coup, including a pastor and a NASA scientist.

In Syria, the U.S. has backed Syrian Kurds that the Turkish government considers an extension of a terrorist group.

Investors in Turkey also are bracing for the possibility of a fine or sanctions against Turkiye Halk Bankasi, a state-run lender whose former deputy chief executive officer was convicted in a New York court of participating in a scheme to help evade sanctions on Iran.