"Our president held a telephone conversation today with the US president. Tayyip Erdogan again put forward a proposal to create a working group on the S-400 air defense systems, which are to be supplied from Russia for our national security needs. Both leaders agreed to hold a meeting on the sidelines of the G20 summit in Japan," he said on Twitter.
Earlier, Turkish Defence Minister Hulusi Akar told the Haberturk channel that Ankara had not received an ultimatum from Washington over the S-400 deal with Russia. His words came in response to reports saying that the US had issued an ultimatum to Turkey, threatening to exclude Ankara from the F-35 fighter jet programme unless it abandoned its plans to purchase Russia's missile defence systems.
The White House spokesperson Judd Deere issued a statement on Wednesday, saying that Trump and Erdogan discussed a number of bilateral issues, including Turkey's planned purchase of the S-400 missile defense system.
Washington has repeatedly claimed that the S-400s are incompatible with NATO military equipment, saying that the Russian air defence systems present a threat to the F-35s. Turkey has responded by stressing that its decision to buy Russian systems "does not target a third country". It also added that it has no plans to abandon its participation in the F-35 programme.
Russia and Turkey signed a $2.5 billion loan agreement for the shipment of a total of four batteries of S-400 systems in late 2017. The first delivery is set for July and will proceed as scheduled, Turkish officials have recently said.