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The United States says it is halting deliveries to Turkey related to the F-35 fighter-jet program in response to Ankara's decision to move ahead with the purchase of Russian air-defense system, RFE/RL reports.

"Pending an unequivocal Turkish decision to forgo delivery of the S-400, deliveries and activities associated with the stand-up of Turkey's F-35 operational capability have been suspended while our dialogue on this important matter continues with Turkey," a Pentagon spokesperson said on April 1.

Washington has been warning Ankara for months that buying the S-400 system would jeopardize its planned purchase of the advanced fighter aircraft.

Ankara has refused to back down on its planned purchase.

On March 29, Turkish Foreign Minister Mevlut Cavusoglu said that his country was committed to the deal to buy the Russian system and was discussing delivery dates.

Russia media reported that Turkey intended to buy four S-400 units for a price of $2.5 billion.

Washington has also said it could withdraw an offer to sell Turkey the U.S. air-defense equivalent -- the Patriot missile system.