Istanbul, June 4 (IANS): Turkish President Recep Tayyip Erdogan on Tuesday vowed to buy Russian-made S-400 missile defence system despite being threatened with sanctions from the US.

"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.

The Turkish leader said that the US was not making an offer as good as the S-400 deal in terms of its Patriots system, Xinhua news agency reported.

"We can take a step for Patriots, if there is a positive offer as Russia made," Erdogan said.

Washington is concerned that the S-400 missiles on Turkish territory could gain valuable intelligence on the technical systems of the US-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 US counterpart Donald Trump have agreed to meet later in June on the sidelines of G20 summit in Japan.