WASHINGTON — The United States has suspended Turkey’s participation in the F-35 Joint Strike Fighter jet program until Ankara cancels its planned purchase of a sophisticated Russian air defense system, the Pentagon said Monday.

“We have … been clear that acquisition of the (Russian) S-400 is not compatible with the F-35,” according to a statement by Lt. Col. Mike Andrews, a Defense Department spokesman. “We very much regret the current situation … but the DOD is taking prudent steps to protect the shared investments made in our critical technology.”

The move is the most direct U.S. response yet to a worsening dispute with Turkey over its purchase of the Russian system, which both the Trump administration and NATO have said would compromise the technology of the new-generation stealth fighter. It comes as the United States and NATO member Turkey are locked in a dispute over Syria, and the day after Turkish President Recep Tayyip Erdogan’s party lost control of major cities in hard-fought local elections.

The Pentagon order stops all “deliveries and activities” related to Turkey’s purchase of 100 of the next-generation stealth fighters. Turkey already has officially taken delivery of two of the planes, but the aircraft have remained in the United States while Turkish pilots are being trained.

Turkey is also one of the co-producers of the $85 million jet, responsible for the manufacture of key components of the fuselage and cockpit. Last year, when U.S. lawmakers first threatened to block Turkey’s participation in the F-35 program, then-Defense Secretary Jim Mattis objected, saying it could delay production lines by up to two years.

Monday’s statement, however, indicated that the Defense Department was prepared to take that step; the Pentagon said it was developing “secondary sources of supply for Turkish-produced parts.”

Last week, a bipartisan group of senators introduced legislation prohibiting the transfer of any F-35s to Turkey. The bill, co-sponsored by Sens. James Lankford, R-Okla., Thom Tillis, R-N.C., Jeanne Shaheen, D-N.H., and Chris Van Hollen, D-Md., said Turkey’s S-400 purchase not only risked NATO security but was a violation of a 2017 law threatening sanctions for any allied purchase of sophisticated Russian technology.

“I’m glad the administration is heeding the bipartisan call in Congress to delay the transfer of F-35 equipment to Turkey to help ensure U.S. military technology and capabilities cannot fall into the hands of the Kremlin,” Shaheen said Monday in a statement.

“Though Turkey is an important U.S. ally, their close relationship with (Russian President Vladimir) Putin and persistent efforts to acquire the Russian S-400 air defense system could seriously compromise our national security,” Shaheen said.

In recent weeks, senior Trump administration officials had speculated that Erdogan was refusing to back down on the S-400 purchase to present a tough national security stance in the run-up to the local elections. They expressed hope that he would soften his position after the vote.

But as recently as Friday, Turkish Foreign Minister Mevlut Cavusoglu said after a meeting with his Russian counterpart, Sergei Lavrov, that “this agreement is a done deal.”

Turkey’s desire for a sophisticated missile defense system predates the start of the civil war in neighboring Syria, but it escalated in the wake of the 2015 Turkish shoot-down of a Russian attack aircraft that entered its territory from Syria.

Russia has been Syrian President Bashar Assad’s primary military supplier and backer in his fight against opposition forces.

NATO partners temporarily rushed Patriot air defense batteries to the Turkish border, and the United States offered to sell Turkey a Patriot system. But Turkey refused, saying that it wanted better terms for the purchase, including technology transfers and a co-production agreement as it has tried to expand its own defense manufacturing industry.

As the Syrian war continued, Turkey and Russia normalized relations, and with Iran launched a diplomatic effort to forge a political solution to the conflict. That effort has sometimes conflicted with a larger, U.S.-backed United Nations initiative to resolve the war.

The Russia-Turkey relationship tightened as the United States expanded its relationship with Syrian Kurdish fighters who led ground forces in the U.S. offensive against the Islamic State in Syria. Turkey considers the Kurdish group to be terrorists allied with its own Kurdish separatists.

As Russia sought to expand its own defense industry as a direct competitor to the United States, it offered the S-400 system on terms that Turkey considered both economically and politically attractive. Last fall, Turkish Defense Minister Hulusi Akar said a contract had been finalized for acquisition and deployment of the Russian long-range air and antimissile defense system – making Turkey the first NATO member state to buy it.

The United States made another Patriot offer in December, but negotiations with the Pentagon have stalled. Turkey has said that delivery of the $2.5 billion S-400 system will begin this year.

NATO and the United States have said that F-35 stealth technology would be compromised if it is placed in proximity to the S-400 system, and warned of “necessary consequences” if Turkey were to proceed with the Russian purchase.

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