WASHINGTON

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."

"You can either have the S-400 or you can have the F-35. You cannot have both," Mark Esper told lawmakers during his confirmation hearing before the Senate Armed Services committee. "Acquisition of the S-400 fundamentally undermines the capabilities of the F-35, and our ability to retain that overmatch in the skies."

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.