Turkey on Sunday dismissed U.S. sanction threats and said it would not renege on a pledge to Moscow to purchase its advanced missile systems.

Washington has previously told its NATO ally that Ankara’s adoption of Russian S-400 missile technology alongside U.S. F-35 fighters would endanger Western defence.

The S-400 is the latest generation surface-to-air defense system developed by Russia as a rival for America’s own Patriot weaponry, and is considered by NATO countries to pose a threat to their combined air operations.

Just last month U.S. Vice President Mike Pence bluntly warned that Turkey is risking the security of the NATO alliance by “making such reckless decisions that undermine our alliance.” The S-400 missiles are not interoperable with the technology NATO countries use, as the alliance requires.

Although the U.S. has already delivered two warplanes under an agreement to sell F-35 fighter jets to Turkey, Congress ordered a delay in future deliveries late last year.

The U.S. Department of State (DOS) approved the sale of a rival $3.5 billion Patriot missile defense system to Turkey last December, the future of which is now in doubt.

A U.S. law furthermore provides for sanctions on any country concluding arms deals with Russian companies.

“The U.S. threats of sanctions shows that they don’t know Turkey,” Vice President Fuat Oktay told Kanal 7 television on Sunday, confirming the new purchase. “The decision on the S-400 has been taken. Once a pact has been signed, one’s word given, Turkey respects it,” he said.

The S-400 is one dispute fuelling tensions between two nations also at odds over U.S. support for Syrian Kurdish militias which Ankara brands as terrorists and Turkish backing for Venezuela.

Turkey had planned to buy 100 F-35A fighter jets, with pilots already training in the United States.

AFP contributed to this report