If Turkey goes ahead and deploys the Russian S-400 system, its radars and missile batteries will not be integrated with those of its NATO allies.

And such a purchase could also be judged a breach of a 2017 US sanctions law designed to punish customers of Russia's arms and aerospace industries.

Washington wants its NATO partners focused on threats from Russian hybrid warfare and on counterterrorism, and sees Ankara's decision as a mistake.

On Friday, a day after he was sworn in as US secretary of state, former CIA boss Mike Pompeo raised the issue in talks with Foreign Minister Mevlut Cavusoglu.

After their talks, a senior State Department official told reporters Pompeo reminded his opposite number Turkey could face sanctions if it bought the S-400.

"He urged Cavusoglu to closely consider NATO interoperable systems," the official said, adding that the meeting had been cordial and workmanlike.

"And there was also a shared desire to find a common way forward on air defence for Turkey," the official said, sounding optimistic.

Immediately before the meeting, however, Cavusoglu had been dismissive.

Asked by a reporter whether Turkey would indeed buy the S-400, he referred to an already announced memorandum of understanding with Russia, asking rhetorically: "You mean in addition to the ones we already bought?"

And, pushed on when Moscow might actually deliver the system, he said simply: "Russia is speeding up the process."

Washington and Turkey are longstanding allies but have fallen out over US support for Kurdish forces in Syria and the asylum granted to exiled cleric Fethullah Gulen.

Last year US lawmakers, seeking to punish Russia for its efforts to undermine Western democracy, passed the Counter America's Enemies Through Sanctions Act.

Under CAATSA, any entity doing business with Russia's state, semi-state and private defence and intelligence sectors could face economic sanctions.