US Vice President Mike Pence has warned Turkey against buying a Russian S-400 anti-aircraft missile system seen in Washington 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.
the US and Europe
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Turkey has the second largest army in NATO, a 29-member military alliance set up to defend against what was at the time of the Soviet Union
Mr Pence also rebuked Germany ̵
Moscow has made public comments on the latest developments
Relation between NATO and Russia has deteriorated over Moscow's occupation of Ukraine's Crimea peninsula and its withdrawal from a key missile treaty.
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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 or does it want to risk the security of partnership with making such decisions that undermine our alliance?"
The US believes that Turkey's The S-400 would be a threat to US F-35 fighter jets.
Washington has already suspended Turkey from its F-35 fighter jet program.
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 canceled
In a tweet, Turkey's vice president later wrote: "The United States must choose.
" our friendship by joining forc es with terrorists to undermine its Nato ally's defense against its enemies? "
Ankara says the S-400 system will help the country to defend itself, as Turkey faces threats from Kurdish rebels and Islamist militants.
What are S-400 systems?
The S-400 " Triumph "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 launched 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