Donald Trump said he has a good relationship with Turkish President Recep Tayyip Erdogan

US President Donald Trump declined Tuesday to criticize Turkey's acquisition of a Russian missile system assailed by the Pentagon and NATO, making no mention of sanctions Washington had threatened over the purchase.

In his first comments since Turkey began taking delivery of the S-400 system last Friday, Trump said Turkey was forced into the move by his predecessor Barack Obama, and that he understood why they opted to buy the Russian missiles.

"I've had a good relationship with President (Recep Tayyip) Erdogan," Trump told journalists.

"It's a very tough situation that they're in and it's a very tough situation that we've been placed in.... With all of that being said, we're working through it -- we'll see what happens."

Trump called it a "complex situation," noting that the Pentagon has suspended Turkey from participating in the NATO F-35 fighter jet production program and from buying a planned 100 F-35s.

"Because they have a system of missiles that's made in Russia, they're now prohibited from buying over 100 planes. I would say that Lockheed isn't exactly happy. That's a lot of jobs," Trump said, referring to the F-35 manufacturer Lockheed Martin.

Congress urges sanctions

Trump's comments appeared to put him at odds with Congress and the Pentagon's view, that placing the S-400 system alongside the F-35 was a threat to the US and NATO air systems.

"Turkey has been a long-standing and very capable NATO ally, but their decision on the S-400 is the wrong one and it's disappointing," Mark Esper, Trump's nominee as secretary of defense, told the Senate Armed Services Committee earlier Tuesday.

"Acquisition of the S-400 fundamentally undermines the capability of the F-35 and our ability to maintain that overmatch in the skies," Esper said.

Esper said he had already told Turkish Defense Minister Hulusi Akar that his policy is, "You can either have the S-400 or the F-35. You cannot have both."

On Friday senior Republican and Democratic senators of the armed services and foreign relations committees urged Trump to implement new sanctions on Turkey and directly "terminate" Turkey's participation in the F-35 program.

"By accepting delivery of the S-400 from Russia, President Erdogan has chosen a perilous partnership with (Russian President Vladimir) Putin at the expense of Turkey's security, economic prosperity and the integrity of the NATO alliance," they said in a statement.

State Department spokeswoman Morgan Ortagus said Tuesday that Secretary of State Mike Pompeo and Trump are reviewing Congressional legislation passed in 2017 that mandates sanctions for any "significant" purchases of weapons from Russia.

Asked if there could be a waiver, Ortagus said: "I think that the secretary and the president will make a decision based off what they are required to do according to US law," without specifying a timeline for the decision.

'Not a fair situation'

But Trump took a softer line as other officials of his administration who had, in the past, expressed strong opposition to the S-400 deal remained silent.

"We have a situation where Turkey is very good with us, very good," Trump said, noting Ankara's release last October of a US Christian pastor held for two years by the government.

"And we are now telling Turkey that because you have really been forced to buy another missile system, we're not going to sell you the F-35 fighter jets.

"And because of the fact he bought a Russian missile, we're not allowed to sell him billions of dollars worth of aircraft. It's not a fair situation," Trump said.

Turkey initially sought in 2009 to buy the US Patriot missile defense system, and a $7.8 billion dollar deal was tentatively approved by the Obama administration.

But Washington balked when Ankara, seeking to boost its own technology base, insisted on Turkey producing some of the system components itself as part of the deal.