"Turkey needs S-400s and its deal has been done," Erdogan said in the western city of Balikesir. "God willing, we will buy them in the shortest time."

In April, Russian President Vladimir Putin agreed to speed up the first deliveries of the S-400s, which had previously been scheduled for the end of 2019 or the beginning of 2020.

The S-400 is the latest generation surface-to-air defence system developed by Russia and is considered by NATO countries to pose a threat to their aircraft.

Such a major purchase of weaponry from Moscow has raised concerns over Turkey's strategic orientation and the compatibility of the systems within the alliance.

It comes at a time of high tension between Turkey and the United States, its NATO ally, over Ankara's detention of an American pastor on terror-related charges.

The crisis has strained ties and sent the already-fragile lira into freefall against the dollar, with US President Donald Trump doubling tariffs on aluminium and steel from Turkey.

That prompted Ankara to impose similar measures on several US products.

Erdogan has remained defiant in the face of US sanctions and said Turkey was not without an alternative, warning it could turn to new allies and markets.

Washington has warned Ankara against delivery of S-400s from Russia, saying any such acquisition would inevitably affect the prospects for Turkish military-industrial cooperation with the United States.

Erdogan on Friday said Turkey "needed cooperation with other countries as much as that with Europe and America."