Russian S-400 air defense mobile missile launching systems drive during a rehearsal for the Victory Day parade in Red Square in central Moscow, Russia, on May 7, 2015. (Reuters File photo)

ISTANBUL, June 4 (Xinhua) -- Turkish President Recep Tayyip Erdogan on Tuesday reaffirmed his resolve to buy Russian-made S-400 missile defense system despite threatened sanctions from the United States.

"There is an agreement. We have determination. It is out of the question to take a step back from it," Erdogan told reporters in Istanbul following prayers.

The Turkish leader also said that the U.S. is not making an offer as good as the S-400 deal in terms of its Patriots system.

"We can take a step for Patriots, if there is a positive offer as Russia made," Erdogan was quoted as saying in local media reports.

Washington is concerned that the S-400 missiles on Turkish territory could gain valuable intelligence on the technical systems of the U.S.-made F-35 jets which Ankara plans to buy, and has threatened to impose severe sanctions on Turkey if the deal with Moscow is followed through.

Erdogan and his U.S. counterpart Donald Trump have agreed to meet later this month on the sidelines of a G20 summit in Japan.