S-400 Triumph anti-aircraft missile complex during a military parade marking the centenary of the formation of the Southern Military District, Rostov-on-Don. Photo:Sputniknew.com

Turkey is a major partner nation in Lockheed Martin's F-35 programme, investing over a billion dollars into the fighter, and voicing its intentions to purchase up to 120 of the advanced fifth-generation warplanes. Washington froze the delivery of F-35s to Turkey over Ankara's decision to buy the S-400, an advanced Russian-made air defence system.

Turkish President Recep Tayyip Erdogan has accused the United States of engaging in 'robbery' over its continued foot-dragging on F-35 deliveries.

"If you seek a customer, and a customer comes forward and makes payments like clockwork, how can you not give that customer their goods? This would be robbery," Erdogan said, speaking to reporters during a state visit to China, Hurriyet has reported.

According to Erdogan, Turkey had already paid $1.4 billion for the planes. However, the four jets formally 'handed over' to Ankara remain stranded at a base in the US, and Turkish pilots were recently barred from training aboard the aircraft.

"We have made an agreement to buy 116 F-35s. We are not just a market, we are also joint producers. We produce some of the parts in Turkey," Erdogan stressed.

Washington has repeatedly threatened to drop Turkey from the F-35 programme and slap Ankara with sanctions unless the country rejects delivery of its Russian-made S-400 air defence systems.  Last week, US President Donald Trump told Erdogan that Washington had treated Turkey "unfairly" in the missile purchase squabble, but blamed his predecessor Barack Obama for the situation.

Erdogan, for his part, has reiterated that he expects the US to hand over the F-35 jets, saying the country has been "transferring payments" as stipulated under the deal.