WASHINGTON, June 26. /TASS/. Turkey’s contract to purchase S-400 missile systems from Russia will severely damage bilateral relations and lead to unilateral US sanctions against Ankara, US Assistant Secretary of State Wess Mitchell said on Tuesday.

The US diplomat told a Foreign Relations subcommittee hearing on US relations with Europe that sanctions against Ankara will be introduced in line with Section 231 of the Countering America's Adversaries Through Sanctions Act (CAATSA), which authorizes restrictive measures against third states that cooperate with Russia’s defense and intelligence sectors.

"We've been clear in all of our communications with the Turkish government that acquisition of the S-400, which we would assess to have occurred when there's actual - an actual delivery of the technology - we've been clear on multiple occasions with the highest levels of Turkish government, there will be consequences, the US diplomat said.

He added that the S-400 contract will change US-Turkish relations to the point when it will be extremely hard to mend them.

"And I think we can't be any clearer than saying both privately and publicly, that a decision on S-400 will qualitatively change the U.S.-Turkish relationship in a way that would be very difficult to repair," the US official said.

"We've also been very clear that across the board, an acquisition of S-400 will inevitably affect the prospects for Turkish military-industrial cooperation with the United States, including F-35," Mitchell continued.

"We believe that we have existing legal authorities that would allow us to withhold transfer under certain circumstances, including national security concerns," he said. "We believe that we continue to have the time and ability to ensure Turkey does not move forward on S-400 before having to take a decision on - on F-35. We're being very clear in our messaging to the Turks that there will be consequences."

Last week, the US Senate adopted the National Defense Authorization Act for fiscal year 2019, which obliges the Secretary of Defense to present a special report regarding the potential purchase of the S-400 air and missile defense system from Russia by the Turkish government. This decision throws into question the delivery of the F-35 fifth-generation fighter jets to Turkey.

An explanatory note for the Senate’s version of the National Defense Authorization Act says that the ban on selling F-35 to Ankara comes due to the imprisonment of US pastor Andrew Brunson in Turkey. The text may also include a call on the US administration to impose sanctions on Turkey in case of its purchase of Russia’s S-400. The final wording will be put together after both houses agree on their versions.

Last week, Turkish Defense Minister Nurettin Canikli slammed the US demand to give up the deal to purchase S-400 air defense missile systems from Russia as ‘blackmail.’ Canikli also said that Turkey "is fulfilling all its commitments" and expected "timely deliveries of F-35 fighter jets" from the United States.

Turkish Foreign Minister Mevlut Cavusoglu told NTV channel last week that Turkey will purchase Russia’s S-400 missile systems and this issue is closed. He stressed that Turkey is not planning to give up this deal despite the decision of the US Senate.

Turkish Foreign Ministry Spokesperson Hami Aksoy said on May 25 his country could take measures against the United States if Washington refused to supply F-35 to Ankara.