U.S. President Donald Trump's nominee to lead the Defense Department said Tuesday that Turkey's acquisition of the advanced Russian S-400 anti-air missile system is "wrong" and "disappointing."

"They have been a long standing and very capable NATO ally, but their decision on the S-400 is the wrong one and it is disappointing," said Esper, testifying before the Senate Armed Services Committee.

"The policy that I have communicated to my counterpart, if confirmed, (Turkey's) defense minister is that you can either have the S-400 or you can have the F-35, you cannot have both," Esper said and added that acquisition of the S-400 undermines the capabilities of the F-35 and Washington's ability to retain that over-match in the skies in the future.

The Trump administration has threatened to expel Turkey from the F-35 fighter program if it acquired the S-400 from Russia, warning the anti-air system could be used by Moscow to covertly attain secret information on the stealth fighter.

The delivery of S-400 components began last week and is ongoing. Turkey's Defense Ministry said a twelfth Russian plane with the anti-air parts landed at Murted airfield outside Ankara on Tuesday.

Following protracted efforts to purchase an air defense system from the U.S. with no success, Ankara signed a contract in 2017 to purchase the S-400s from Russia.

The U.S. halted in June training for Turkish pilots as a step in winding down Turkey's participation in the F-35 program.

Washington has also threatened sanctions over Turkey's purchase, with Ankara responding that any sanctions would be met in kind.

Esper had been acting as defense secretary since last month when Patrick Shanahan stepped down amid revelations about his family's past that were brought to light during his nomination process.

Navy Secretary Richard Spencer took over the Defense Department on an acting basis while Esper goes through his nomination process.