The Department of Defense announced Monday that it "suspended" deliveries of F-35 fighter jet parts and manuals to Turkey over the Middle Eastern country's decision to purchase a Russian air defense system over Washington's objection.

The U.S. had agreed to sell 100 of its latest, fifth-generation F-35 fighters to Turkey, initially planning to deliver the first two aircraft to Turkey in June. However, top government officials repeatedly have threatened to stop the sale if Ankara did not abandon efforts to buy the Russian S-400 system.

"The United States has been clear that Turkey's acquisition of the S-400 is unacceptable," said acting Pentagon spokesman Charles Summers Jr., who added: "[U]ntil they forgo delivery of the S-400, 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."

The U.S. move came just three days after Turkey's foreign minister said his country, a NATO ally, was committed to the deal to buy the Russian system and was discussing delivery dates.

"As a principle, it is contrary to international laws for a third country to oppose an agreement between two countries," Mevlut Cavusoglu said at a joint news conference with his Russian counterpart, Sergey Lavrov. "We are committed to this agreement. There can be no such thing as selling to a third country. We are buying them for our own needs."

Cavusoglu also insisted Turkey had met all of its obligations concerning the F-35 program.

The U.S. and other NATO allies have complained repeatedly about the purchase, saying it was not compatible with other allied systems and would represent a threat to the F-35.

Last month, Gen. Curtis M. Scaparrotti, the outgoing Supreme Allied Commander in Europe, told lawmakers on Capitol Hill that "my best military advice would be that we don't then follow through with the F-35, flying it or working with an ally that is working with Russian systems, particularly air defense systems."

Officials also have expressed concerns that Turkey's acquisition of both U.S. and Russian systems could give Moscow access to sophisticated American technology and allow it to find ways to counter the F-35.

"There is great concern that if they acquire that missile system and have the F-35 airplane, the Russians will have a window into the technology that’s used to produce the F-35," Sen. Jeanne Shaheen, D-N.H., told Fox News on Tuesday.

Pentagon leaders have warned that ending Turkey's participation in production likely would force other allies to take on that role and could delay aircraft delivery.

Summers said Monday that the Pentagon "has initiated steps necessary to ensure prudent program planning and resiliency of the F-35 supply chain. Secondary sources of supply for Turkish-produced parts are now in development.

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"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," he said.

U.S. leaders have pressed Turkey to buy an American-made air defense battery, and in December the State Department approved the sale of a $3.5 billion U.S. Patriot system to Ankara.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.