Turkish President Recep Tayyip Erdogan has rejected the Pentagon's ultimatum on Ankara's purchase of the S-400, Sputnik reported.

"The S-400s we're buying are not connected to the security of NATO, the United States or the F-35 in any way," Erdogan said, speaking at a rally in Diyarbakir on Saturday, according to Turkish television channel TRT.

"The reasons for our purchase of these systems are obvious," the president added, stressing that Ankara had nothing to hide regarding the intended use of the S-400s.

Erdogan expressed hope that the problems in relations between Turkey and the US over Ankara's purchase of the Russian air defence system could "be resolved in the same way as it has on other issues."

Erdogan's remarks follow on comments Friday by US Department of Defence spokesperson Charlie Summers that "there would be grave consequences in terms of" the US-Turkish "military relationship, and the Patriots and the F-35s" if Ankara went ahead with the delivery of the S-400s. "They will not get the F-35s if they take the S-400," Summers said.