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 lawmakers 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.