The NATO alliance is worried Turkey's intent to deploy Russia's S-400 anti-aircraft missile system could provide Moscow with a trove of intelligence on Western war equipment.

The planned purchase has become a major source of friction, and US lawmakers have warned Ankara could face sanctions if it goes ahead with the purchase of the S-400.

"We are concerned that by purchasing these systems from the Russians, it will be supportive of some of the least good behavior that we have seen from them," said Tina Kaidanow, acting assistant secretary of the State Department's Bureau of Political Military Affairs.

She added that the US wants to ensure the systems its allies buy "remain supportive of the strategic relationship between us and our allies."

"In the case of Turkey, that, in our view, is Patriots and we're trying to give the Turks some understanding of what we can do with respect to Patriots."

The US last month warned it could block the delivery of F-35 stealth jets to Turkey if Ankara buys Russia's S-400 system.