Washington has warned its NATO ally for months that Ankara's adoption of Russian S-400 missile technology alongside US F-35 fighters would pose a threat to the jets and endanger Western defence.

The US has said it will halt a joint F-35 programme with Turkey if it acquires the Russian missile defence system. A US law furthermore provides for sanctions on any country concluding arms deals with Russian companies.

"The US threats of sanctions shows that they don't know Turkey," Vice President Fuat Oktay told Kanal 7 television.

"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 purchase is one dispute fuelling tensions between two nations also at odds over US support for Syrian Kurdish militias which Ankara brands as terrorists and Turkish backing for US foe Venezuela.

Ankara said the first deliveries of the S-400 are scheduled for June or July.

Last month, after repeated warnings, the United States said Turkey's decision to buy the S-400 system was incompatible with it remaining part of the emblematic F-35 jet programme.

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

Washington has placed a freeze on the joint manufacturing operations with Turkey, and suggested Ankara might be able to obtain a US missile defence system if it forgoes the one on offer from Moscow.