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US President Donald Trump on Tuesday refused to condemn Turkey's purchase of a controversial Russian missile defence system, saying it was "not fair" to either Washington or Ankara that Washington could no longer sell billions of dollars worth of F-35 fighter jets to Turkish President Recep Tayyip Erdogan.

Turkey began receiving delivery of the Russian S-400s last week amid continuing tensions over its purchase of the missile defense system.

Already strained relations between Washington and Ankara have deteriorated in the past months as Erdogan refused to back down over Turkey's planned purchase of the S-400s. 

The US repeatedly threatened to implement sanctions and kick Turkey out of the F-35 programme in response to the purchase.

Washington is concerned the S-400s could be used to gather data on F-35 stealth capabilities if Turkey has both, and threatened a number of measures against Ankara, including cancelling the sale of the jet fighter.

Despite those warnings, the US president has often struck a sympathetic tone in conversations with Erdogan, blaming former President Barack Obama for allegedly forcing Ankara into the controversial deal by "refusing" to sell the US' own Patriot missiles to Turkey.

The Patriot sale derailed after Obama denied Erdogan a technology handover along with the missiles. Such a handover could allow Turkey to develop and produce its own missiles, and a partial handover has been granted to Ankara by the Kremlin.

Trump on Tuesday confirmed that the US would no longer be going through with the sale of its F-35 fighter jets to Turkey, but again expressed regret and sympathy with Turkey over the crisis.

"We are now telling Turkey that 'because you have really been forced to buy another missile system, we are not going to sell you the F-35 fighter jets'," the president said, according to Bloomberg. "It is a very tough situation that they are in, and it's a tough situation that we have been placed in, the United States."

Trump described the situation as "not fair", adding that F-35 manufacturer Lockheed was also "not exactly happy". The president also lamented the loss of jobs over the failed deal.

The US president has not yet made clear whether Turkey will face sanctions over the S-400 purchase.

"I don't believe Mr. Trump thinks the same as others under his management," Turkish President Erdogan said on Sunday, claiming that Trump - unlike many US officials - did not support sanctions.

As Turkey marked the third anniversary of a failed coup attempt against Erdogan's government on Monday, the Turkish president hailed the delivery of the S-400 systems.

He added that the S-400s would be fully operational by April next year and that Turkey aims to begin joint-production of the defence systems with Russia in the future.