Relations between the NATO allies deteriorated sharply amid Washington's threats to sanction Ankara over its purchase of Russian air defence systems and to renege on its agreement to sell the country Lockheed F-35 warplanes.

President Donald Trump has admitted that the US has treated Turkey in an unfair way over the S-400 missile system squabble, but blamed his predecessor Barack Obama for the situation.

"It's a problem, there's no question about it…But [the Turkish president] was prohibited from buying until he said he'd bought something else, and then as soon as he buys something else everybody says 'ok you can buy'. You Can't do business that way. Turkey has been a friend of ours and we've done great things together," Trump said, speaking to President Recep Tayyip Erdogan on the sidelines of the G20 summit on Saturday.

"You can't treat people that way, like the Obama administration did," Trump continued.

Asked whether Washington would slap Ankara with sanctions for purchasing the S-400 instead, as US officials have repeatedly threatened, Trump said "We're looking at it."

Reiterating that Erdogan "wanted to buy the Patriot" but was refused, Trump said he had "become friendly" with the Turkish president, and "you have to treat people fairly…And I don't think he was treated fairly."

Russia and Turkey penned a $2.5 billion contract on the delivery of four battalion sets' worth of S-400s to Turkey in December 2017, nearly a year into Trump's presidency. In December 2018, the US cleared a $3.5 billion Patriot missile deal for Turkey, but Ankara has yet to accept, saying the terms proposed by the US simply aren't as good as its deal with Russia. Turkish officials have said that they are still looking at buying the US systems, but not in place of their S-400s, whose deliveries are expected to start in July.

© Sputnik / Sergey Pivovarov 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

S-400s on Schedule, Turkey Still Wants Its F-35s

Speaking with Russian President Vladimir Putin on Saturday on the sidelines of G20 summit, Erdogan confirmed the S-400 delivery was set to proceed as planned, adding that technology transfer was an important "priority for our cooperation."

Later, speaking at a summit briefing, the Turkish president said he still expects the US to sell its F-35 jets to Ankara despite recent threats to terminate deliveries. "We still expect jet deliveries to go ahead. We have been transferring payments," Erdogan said. On the question of the S-400 deal, he said the subject was "closed entirely," adding that US sanctions on Turkey over the Russian missile system purchase were "unthinkable."

Turkey has been a major partner nation in Lockheed Martin's F-35 programme, manufacturing several key components for the planes, investing over $1.25 billion into R&D for the plane, and voicing plans to buy up to 120 of the advanced jet fighters.

'Good Cop' Trump

Commenting on President Trump's remarks, Aydin Sezer, a columnist for Turkish media resource Medya Gunlugu and head of the Ankara-based Turkey & Russia Centre of Studies think-tank, said he wasn't sure the US president could stop anti-Turkish sanctions over the S-400 deal even if he wanted to.

"I think Trump is trying to play the role of good cop," Sezer said. "The White House commented immediately...that the issue is still on the table and expressed that Turkey and USA should work together to develop joint defence system which is compatible with NATO systems. I think that the delegations of the two countries will continue to [search for] a solution to the S-400 issue. And we should consider that Trump is not able to prevent Turkey from the sanctions. He has no authority to do that," the analyst concluded.