Turkish President Recep Tayyip Erdogan has reaffirmed his commitment to the S-400 deal with Russia, vowing to retaliate if the United States stands in the way.
Speaking at a press conference on Thursday, Erdogan said he didn't see any possibility of the United States slapping sanctions on Turkey under the Countering America's Adversaries Through Sanctions Act.
Describing the purchase of the much-vaunted S-400s as a "done deal", he urged Washington to "think very carefully" if it still decided to go ahead with the sanctions, pledging to respond in kind.
He also threatened that Turkey would appeal to international courts to have its investments in the F-35 programme refunded if the US "does something wrong".
According to a 2017 contract, worth $2.5 billion in Russian roubles, Moscow is set to supply Ankara with four batteries of the cutting-edge S-400 air defense systems, which are widely considered among the best of their kind in the world.
President Erdogan announced this week that the deliveries would start as early as the first half of July.
However, the United States, Turkey's NATO ally, has opposed the move, saying that the S-400s would compromise its fifth-generation fighter, the F-35, that Turkey is also looking to acquire.
Washington is worried that the Russian systems could be used to gather useful data on the F-35s − such as its advanced stealth capabilities − and has threatened to remove Turkey from the programme.
Ankara has argued that the purchase of the S-400s is solely meant to meet the country's defence needs and poses no threat to the fighter jets.