Turkey has been given a deadline of the end of July to choose between buying US fighter jets and Russian anti-aircraft missile systems, APA reports citing BBC.
Acting US Defence Secretary Patrick Shanahan set out the ultimatum in a letter to his Turkish counterpart, Hulusi Akar.
Turkey, he said, could not have both America's F-35 advanced fighter jets and Russia's S-400 systems.
The two Nato allies have been locked in a row over the S-400 for months.
America argues that the Russian systems are both incompatible with Nato defence systems and pose a security threat, and wants Turkey to buy its Patriot anti-aircraft systems instead.
Turkey, which has been pursuing an increasingly independent defence policy, has signed up to buying 100 F-35s, and has invested heavily in the F-35 programme, with Turkish companies producing 937 of the plane's parts.