If by July 31 Turkey does not give up on the S-400 system, Turkish pilots currently training in the United States on the F-35 will be expelled, and agreements with Turkish firms sub-contracted for manufacturing the F-35 stealth warplane will be cancelled, Ellen Lord, Under Secretary of Defense for Acquisition and Sustainment, told reporters.

Lord said the deadline "will allow sufficient time for Turkish personnel associated with the F-35 program to be reassigned and depart the United States... to facilitate an orderly cessation of Turkish participation."

She justified the US ultimatum by the fact that Turkey, a NATO ally of the United States, had already sent its personnel to Russia to start training with the S-400.

On Tuesday Turkish President Recep Tayyip Erdogan said his country was "determined" to proceed with the Russia deal.

US officials have said they expected Turkey to opt for the American Patriot system instead, arguing that would allow the F-35 program to continue.

Turkey plans to buy 100 US F-35s.

Erdogan said he told the US that Ankara would buy Patriots only if Washington's conditions of delivery were as positive as Moscow's.

"But unfortunately we haven't received a positive proposal from the American side on the subject of Patriots like the S-400s from Russia," he said.

Acting US Defense Secretary Patrick Shanahan said he had sent a letter to his Turkish counterpart Hulusi Akar informing him of Washington's decision.

The US offer for the Patriots was "very competitive," Shanahan told reporters.