US Vice-President Mike Pence has warned Turkey against buying a Russian S-400 anti-aircraft missile system that Washington sees as a threat to US jets.
He said Turkey "must choose" between remaining a key Nato member or risk the security of that partnership "by making such reckless decisions".
Turkey responded that the purchase of the advanced system was a done deal.
Ankara has been establishing closer links with Russia after recent souring of its ties with the US and Europe.
Turkey has the second-largest army in Nato, a 29-member military alliance set up to defend against what was at the time the Soviet Union.
Mr Pence also rebuked Germany - another key Nato member - for not spending enough on its defence.
Moscow has made no public comments on the latest developments.
Relations between Nato and Russia have deteriorated over Moscow's occupation of Ukraine's Crimea peninsula and its withdrawal from a key missile treaty.
An unreliable member of the club?
By Jonathan Marcus, BBC defence and diplomatic correspondent
The purchase of the Russian-built S-400 air defence system by Turkey - a Nato member - poses a variety of problems, both practical and political.
The system would not be capable of integration into Nato's wider air defence system - problematic given Turkey's strategic location on the alliance's eastern flank.
Turkey is also purchasing the most advanced warplane in the US arsenal - the F-35. Washington has suspended deliveries of the aircraft pending resolution of the missile problem.
The US fears that if Turkey operates both the F-35 and the S-400, crucial data might be gathered by the Russians which would enable them to better understand the aircraft's characteristics and thus how best to defeat it.
At a political level this row is just another in a raft of differences pitting Washington and Ankara against each other, reinforcing the growing perception that Turkey is increasingly an unreliable member of the Nato club.
What did Pence say?
Speaking at a gathering in Washington to mark Nato's 70th anniversary, Mr Pence said: "Turkey must choose. Does it want to remain a critical partner in the most successful military alliance in history, or does it want to risk the security of that partnership by making such reckless decisions that undermine our alliance?"
The US believes that Turkey's purchase of the S-400 would be a threat to US F-35 fighter jets.
Washington has already suspended Turkey from its F-35 fighter jet programme.
The US has also been pushing for Turkey to buy America's Patriot missiles instead.
Senior Nato officials have repeatedly stated that the Russian system is not compatible with the alliance's equipment.
How did Turkey react?
Turkish Foreign Minister Mevlut Cavusoglu repeated that the deal with Russia - thought to be worth about £2.5bn (£1.9bn) - would not be cancelled.
In a tweet, Turkey's vice-president later wrote:
The United States must choose. Does it want to remain Turkey’s ally or risk our friendship by joining forces with terrorists to undermine its NATO ally’s defense against its enemies?— Fuat Oktay (@fuatoktay06) April 3, 2019
Ankara says the S-400 system will help the country to defend itself, as Turkey faces threats from Kurdish rebels and Islamist militants.
What is the S-400 system?
The S-400 "Triumf" is one of the most sophisticated surface-to-air missile systems in the world.
It has a range of 400km (250 miles), and one S-400 integrated system can shoot down up to 80 targets simultaneously.
Russia says it can hit aerial targets ranging from low-flying drones to aircraft flying at various altitudes and long-range missiles.
How the S-400 system works
- Long-range surveillance radar tracks objects and relays information to command vehicle, which assesses potential targets
- Target is identified and command vehicle orders missile launch
- Launch data are sent to the best placed launch vehicle and it releases surface-to-air missiles
- Engagement radar helps guide missiles towards target