Turkey's push to buy the systems has raised questions among NATO allies over their compatibility with alliance equipment as well as concerns over the burgeoning relationship between Erdogan and Russian President Vladimir Putin.

"We agreed a deal with Russia on the S-400, so for us to turn back from the deal is out of the question. This is a done deal," Erdogan said, quoted by CNN Turk broadcaster.

The United States in December approved the sale of $3.5 billion in missiles to Turkey, which followed Washington anger over Turkey's intention to buy Russian systems.

Turkey was "open" to buying US Patriot missiles, Erdogan told Turkish journalists on board his plane from the southern Russian city of Sochi after a three-way summit on Syria with his Russian and Iranian counterparts.

"But this sale must serve the interests of our country. To this end, joint production, credit and early delivery are of vital importance," Erdogan added.

The Turkish leader said the US administration "looked positively" at early delivery but "said nothing regarding joint production and credit".

Erdogan said work continued for the systems to be delivered in July as promised before.

Washington has warned Turkey the S-400s purchase jeopardised participation in the F-35 fighter jets programme. US officials have said Ankara could even face sanctions on defence purchases under US law if it goes ahead.

But Ankara has said the two defence systems are not seen as an alternative for the other.

The S-400 deal is one of the key symbols of the warm relationship enjoyed by Erdogan and Putin, who have also worked closely on finding a political solution to the Syrian war.