The Kremlin said that the sale of a Russian missile system to Turkey should go ahead despite protest from the United States.
According to a CNBC exclusive Tuesday, Turkish officials have until the first week of June to cancel the purchase of Russia's S-400 missile defense system and instead buy Raytheon's U.S.-made Patriot missile.
Reuters reported Wednesday that Kremlin officials described the ultimatum from the U.S. government as unacceptable and that Moscow considers the sale of its missile system to Ankara as an irrevocable agreement.
Kremlin spokesman Dmitry Peskov told reporters: "We regard this extremely negatively. We consider such ultimatums to be unacceptable, and we are going on the many statements made by representatives of Turkey's leadership headed by President Erdogan that the S-400 deal is already complete and will be implemented."
U.S. officials see Ankara's purchase of the missile system as a pivot toward warmer ties with Moscow. Turkey's integration of the Russian S-400 system is also viewed as a direct threat to the security of Lockheed Martin's F-35 fighter jet program.
If Turkey refuses to comply with Washington's demands, then it faces losing their supply chain position within the F-35 program. In April, Turkish President Recep Tayyip Erdogan claimed the U.S. jet program would collapse without Turkey's input.
According to CNBC's own sources, the U.S. State Department's offer to Ankara is the final one and will not be up for negotiation.
The United States has already ratcheted up the pressure on Turkey by halting delivery of equipment related to the F-35 stealth fighter.
NATO member Turkey is currently due to receive two of the jets later this year and has more than 100 on order.