Turkey's push to buy the S-400s has further strained already tense relations with the United States which has repeatedly warned Ankara of the risks including sanctions as a result of the purchase.

"There is absolutely no question of (Turkey) taking a step back from the S-400s purchase. That is a done deal," Erdogan said in Istanbul.

"There will be joint production of the S-500 after the S-400," Erdogan told an audience of young people asking questions.

Ties between NATO allies Turkey and the US have frayed over multiple issues including American support for a Syrian Kurdish militia viewed as terrorists by Ankara and the US failure to extradite a Muslim preacher blamed for the 2016 coup attempt against Erdogan.

Washington says the deal with Moscow is a threat to Western defence and in April suspended deliveries of the F-35 stealth fighter jet to Turkey in a bid to halt the purchase.

Turkish pilots are in the US receiving training on the F-35s, manufactured by Lockheed Martin. Turkey is expected to buy 100 of the jets in total.

Erdogan said Turkey conducted technical studies amid US concerns over the compatibility of the S-400s and the F-35s but found there were no issues.

He also insisted "sooner or later" Turkey would receive the F-35 jets.

Despite the threat of sanctions, Erdogan repeated that the S-400s were expected to be delivered in July, "but this could be brought forward", he added.

raz/pma

LOCKHEED MARTIN