WASHINGTON (AP) — Army Secretary Mark Esper, President Donald Trump’s nominee to become secretary of defense, told his Senate confirmation hearing Tuesday he is troubled by Turkey’s decision to defy the United States by acquiring Russian-made air defenses.

“It is very disheartening to see how they have drifted over the past several years,” Esper said, citing the Turkish government’s purchase of the S-400 air defense system that the Trump administration has said is likely to trigger U.S. economic sanctions and jeopardize Turkey’s role in the NATO alliance, whose primary adversary is Russia.

At the White House, Trump took a more conciliatory approach to Turkey, asserting that it chose to buy from Russia because the Obama administrations refused to sell the Patriot air defense system “under any circumstance.” In fact, talks between the U.S. and Turkey on a potential Patriot deal during the Obama years faltered on Turkey’s insistence on technology transfer rights that would have allowed it eventually to make the missiles themselves. This ran afoul of the U.S. manufacturer’s propriety interests in addition to any national security concerns.

Turkey began taking delivery of S-400 components in recent days, but the Trump administration has yet to fully explain its intended response. Trump told reporters Tuesday that the U.S. would not sell the F-35 to Turkey, but he did not address other aspects of the issue, such as the expected removal of Turkey as a supplier of F-35 components.

“We are now telling Turkey that because you have been forced to buy another missile system we’re not going to sell you the F-35,” Trump said. Pentagon officials have said Turkey was offered favorable terms to buy a U.S.-made air defense system, the Patriot, but turned it down in favor of the Russian system despite strong U.S. objections.

The Pentagon has repeatedly said the S-400 purchase would mean Turkey’s indefinite suspension, and potentially its permanent removal, from the U.S. F-35 stealth fighter program as a supplier of numerous components. It also has suspended a program of F-35 flight training for Turkish student pilots and instructor pilots at Luke Air Force Base in Arizona and given those personnel a July 31 deadline to leave the United States.

The U.S. governmen’ts concern is that the S-400 could be used by Turkey to gather data on the capabilities of the F-35, and that the information could end up in Russian hands.

Asked about Turkey’s S-400 decision, Esper told the committee it was “the wrong one, and it’s disappointing.”

Later he went further, saying that although Turkey has long been a reliable NATO ally, it is now headed in the wrong direction. He said he recently told Turkey’s defense minister, “You can either have the S-400 or you can have the F-35. You cannot have both. Acquisition of the S-400 fundamentally undermines the capabilities of the F-35.”