Ankara's desire to buy the S-400 has been a major source of contention between NATO allies Turkey and the United States, which has threatened sanctions after months of warnings.

"We have made an agreement (with Russia). We are determined," Erdogan was quoted as saying by the official Anadolu news agency.

"There is nothing like backtracking from that," he told journalists after prayers at an Istanbul mosque.

Last week, a top Pentagon official said the consequences would be "devastating" for Turkey's joint F-35 fighter programme and its cooperation with NATO if the country went ahead with plans to buy the Russian anti-aircraft weapon system.

Kathryn Wheelbarger, Acting Assistant Secretary of Defense for International Security Affairs, said the planned purchase would damage Turkey's ability to work with the Western alliance, and force Washington to hit the country with sanctions against arms deals with Russia.

She said the US administration, even if it does not want to punish Turkey for the purchase, could be forced to do so by a Congress unsympathetic to Ankara.

- S-400s over US Patriots-

US officials said they expect Turkey to opt for the American Patriot missiles instead, arguing that would then allow the F-35 programme to continue.

Turkey plans to buy 100 US F-35s, and some Turkish pilots have already started training with counterparts in the US.

Erdogan said Tuesday he told the US that Ankara would take steps to buy the Patriots only if its conditions of delivery were as positive as Russia's.

"But unfortunately we haven't received a positive proposal from the American side on the subject of Patriots like the S400s from Russia," he added.

Turkey has defied mounting pressure from its NATO allies and said the purchase from Moscow was a "done deal."

Erdogan on Wednesday spoke with US President Donald Trump by phone and, according to the Turkish leader's office, they discussed Ankara's previous offer to form a "joint working group" on the missile system.

The two leaders are due to meet on the sidelines of the G20 summit in Osaka, Japan, at the end of June.

In addition to the issue of the S-400, Turkish-US ties are already under strain over US support for a Syrian Kurdish militia in Syria -- viewed as "terrorists" by Ankara.