Turkey's agreement to buy the Russian S-400 system has further tested its relations with Washington, stoking US concern about Erdogan's burgeoning relationship with Russian leader Vladimir Putin.

United States officials have repeatedly warned of the risks the Russian system would pose to Western defences and the lack of interoperability with NATO hardware.

Relations between NATO allies US and Turkey were already on shaky ground over issues including US support to a Syrian Kurdish militia viewed by Ankara as terrorists.

The US said on Monday it was halting all deliveries and joint work with Turkey on the F-35 fighter jet programme if Ankara insisted on the deal.

"The S-400 process is complete," Erdogan said on Friday, adding that the first delivery of the system would be in July.

Erdogan criticised "really wrong" statements coming from the US over the S-400 purchase and suggested NATO allies should not sanction each other.

Turkey could be sanctioned by Washington under the Countering America's Adversaries Through Sanctions Act (CAATSA), which rules that any entity doing business with Russia's state and private defence and intelligence sectors risks sanctions.

The S-400 is an anti-missile and anti-aircraft weapon system, similar to the US Patriot.

Washington has suggested Turkey could opt for Patriot missiles instead.

But Erdogan said although Ankara was keen to buy the US-produced missiles, "America was unfortunately not giving Patriots to us on the same terms" as Russia.

He has previously said Turkey wanted joint production, credit and early delivery in any Patriot deal, but said Washington did not look favourably on the first two terms.

Despite the US suspension, Erdogan said three F-35 jets had been delivered, though they remain in the US. "The fourth will be delivered in a few weeks."

He said that Turkish pilots were still training in the US.

Erdogan will be in Russia on Monday for talks with Russian counterpart Putin, his third visit to the country this year.