Turkish President Recep Tayyip Erdogan is in talks to get Russia to supply Ankara with its latest S-400 surface-to-air missile system.
Pentagon spokesman Navy Captain Jeff Davis said the Pentagon had concerns over its NATO ally's purchase of the Russian technology because it might not work with other equipment used by the 29-nation alliance.
"Generally speaking it's a good idea for allies to buy inter-operable equipment," Davis said.
"With any ally, with any partner with whom we inter-operate ... we want them to buy things and invest in things that will further invest in our alliance."
Turkish media have in recent days reported that the deal is being finalized with Russia.
If it is completed, it would be a significant step after the two countries repaired relations damaged by Ankara's downing of a Russian warplane on the Syrian border in 2015.
The S-400 system has a range of about 400 kilometers (250 miles) and is designed to shoot down enemy aircraft.
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