The United States has put a hold on delivering F-35 fighter aircraft equipment to Turkey over Ankara’s "unacceptable" plans to buy a Russian air defense system, according to the Pentagon.

Until Turkey forgoes the delivery of Russia’s S-400 long-range air defense system, "the United States has suspended deliveries and activities associated with the stand-up of Turkey's F-35 operational capability. Should Turkey procure the S-400, their continued participation in the F-35 program is at risk," chief Pentagon spokesman Charles Summers said in a statement released Monday.

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"The United States has been clear that Turkey's acquisition of the S-400 is unacceptable. . . .We very much regret the current situation facing our F-35 partnership with Turkey, and the DoD is taking prudent steps to protect the shared investments made in our critical technology."

Reuters first reported on Monday that U.S. officials relayed to their Turkish counterparts that they will not get any more shipments of F-35-related equipment, as they have been canceled. The NATO ally needs the equipment to prepare to receive the aircraft, which has a tentative November delivery date.

The move is the latest effort from Washington to pressure Turkey not to buy the S-400.

Two sources familiar with the situation told Reuters that U.S. officials relayed to their Turkish counterparts that they will not get any more shipments of the equipment, as they have been canceled. The NATO ally needs the equipment to prepare to receive the aircraft, which has a tentative November delivery date.

The move is the latest effort from Washington to pressure Turkey not to buy Russia’s S-400 long-range air defense system.

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The United States and other NATO allies have warned the S-400 system will not work with other NATO defense systems, and threatened Turkey with U.S. sanctions against those who do business with Russia’s defense industry.

But Turkish President Recep Tayyip Erdoğan has not backed down, saying his government will receive the S-400s in July.

“It is out of the question for us to revoke the S-400 deal,” Erdoğan said in March.

Defense officials this past month have said they would recommend freezing delivery of the Lockheed Martin-made F-35 to Turkey, which is supposed to eventually get 116 of the fifth-generation fighter jets, should it proceed with the S-400 purchase.

Joint Chiefs of Staff Chairman Gen. Joseph Dunford said the United States will “have a hard time reconciling” delivery of the F-35 if Turkey follows through on buying the Russian system, which U.S. officials fear could be used to gather information on the most advanced U.S. aircraft.

The canceled shipments also follows reports late last month that Washington was figuring out whether it could remove Turkey as a partner in making the F-35. Parts of the jet are built in the country, and there is an engine overhaul depot in the city of Eskisehir.

Summers said the Defense Department has initiated the necessary steps to ensure "resiliency of the F-35 supply chain," should Turkey be stripped of its partnership, adding that "secondary sources of supply for Turkish-produced parts are now in development."

Washington has also offered Turkey the U.S.-made Patriot anti-missile system at a discount deal, an attempt to sway the country to abandon the S-400 buy.

The issue will likely be discussed further this week, with Turkish Foreign Minister Mevlut Cavusoglu’s planned visit to Washington for a NATO summit.

U.S. lawmakers, meanwhile, also made moves to prevent the F-35 from reaching Turkey, with a bipartisan group of senators last week introducing a bill that would prohibit the United States from transferring the aircraft to Turkey until Ankara abandons its plans to buy the S-400.

—Updated at 5:11 p.m.