Kathryn Wheelbarger, Acting Assistant Secretary of Defense for International Security Affairs, said that Ankara's planned purchase of the S-400 system would damage Turkey's ability to work together with the Western alliance, and force Washington to hit the country with sanctions.

"Completion of this transaction would be devastating, not only to the F-35 program, on which the West has placed its modernized integrated air capability, but it would potentially rupture Turkish interoperability with NATO, a key aspect of the defense of the alliance," Wheelbarger told an audience at the Atlantic Council in Washington.

"Let's be clear. The S-400 is a Russian system designed to shoot down an aircraft like the F-35, and it is inconceivable to imagine Russia not taking advantage of that collection opportunity."

Wheelbarger said the US believes Turkey is pursuing the deal to get Russia's support against Kurdish rebels along its border with Syria.

But she warned Ankara that Russia is not a reliable partner over the long term, doesn't back its military sales with maintenance and other support, and is simply trying to undermine NATO cohesion.

"Once you introduce Russian systems, it really undermines our ability to continue helping them to defend themselves," she said.

She said that the Trump administration, even if it does not want to punish Turkey for the purchase, could be forced to do so by a Congress unsympathetic to Ankara.

"If we don't do the sanctions, they said, they'll just pass another law and make us do it," she said, referring to the US legislature.

Turkish President Recep Tayyip Erdogan has rejected US pressure, calling the purchase a "done deal."

Defense Minister Hulusi Akar said last week that Turkey had already sent personnel to Russia for training.

On Wednesday US President Donald Trump spoke by phone with Erdogan, in which they discussed the S-400 purchase, Erdogan's office said.

The office said they discussed Erdogan's previous offer to form a "joint working group" on the missile system.