The United States warned Turkey against moving ahead with plans to buy a sophisticated Russian missile defence system that the Pentagon believes would threaten its advanced F-35 fighter aircraft.
The State Department made the remarks on a day when the head of US European Command spoke to politicians on Capitol Hill and said Turkey should reconsider its plan to buy the S-400 from Russia this year.
"We've clearly warned Turkey that its potential acquisition of the S-400 will result in a reassessment of Turkey's participation in the F-35 programme, and risk other potential future arms transfers to Turkey," said deputy spokesman Robert Palladino on Tuesday.
The US agreed to sell 100 of its latest fifth-generation F-35 fighters to Turkey and has so far delivered two of the aircraft. But Congress last year ordered a delay in future deliveries.
NATO nations - particularly the US - view Turkey's purchase of the Russian-made equipment with suspicion. Ankara says it needs alternative missile defences to counter regional threats.
Russia's S-400 is a massive upgrade to the S-300 missile defence system, its predecessor. Because of its capabilities, countries such as China, Saudi Arabia, Turkey, India and Qatar have said they want to buy the S-400.
Those stating such intentions, however, have been threatened with some kind of diplomatic retaliation from the US or other NATO nations.
The S-400 can track a large number of potential targets, including stealth targets such as the US F-35 fighter jet.
Other advantages included its high mobility, meaning it can be set up, fired and moved within minutes.
