MOSCOW (Sputnik) - The Kremlin has an extremely negative stand on all ultimatums, including Washington's calls on Turkey to abandon the deal on buying Russian S-400 air defence systems, Kremlin spokesman Dmitry Peskov said on Wednesday.

"We are extremely negative about it. We believe that such ultimatums are inadmissible," Peskov told reporters, when asked about the Kremlin's stand on such statements.

Earlier in the day, Turkish Defenсe Minister Hulusi Akar said that a group of Turkish military personnel had arrived in Russia to learn operating the air defence systems despite numerous US threats.

US CNBC broadcaster reported on Tuesday, citing sources, that the United States had given Turkey two weeks to cancel its deal with Russia. Otherwise, Washington will reportedly remove Ankara from the Lockheed Martin's F-35 program.

Washington has threatened Ankara with sanctions for its planned acquisition of S-400s and has repeatedly said it may delay or cancel the process of selling its F-35 aircraft to Turkey. Ankara, in its turn, has said that purchase of military equipment is its sovereign affair and has ruled out the possibility of abandoning its plans.

READ MORE: Turkey RULES OUT Russian-Made S-400 Integration With NATO Infrastructure

Moscow and Ankara signed in December 2017 a loan agreement for the delivery of S-400 systems. The first delivery of S-400s to Turkey is expected in June. Turkish cooperation with Russia on S-400 deliveries has been strongly criticized by NATO and the United States, which has cited security concerns and incompatibility of S-400s and NATO's air defence systems.