The move to buy the Russian S-400 system over America's Patriot equivalent has been a major source of tension between the NATO allies, leading to threats of US sanctions.

In his call with Trump, Erdogan reiterated the offer to form "a joint working group on Turkey's plan to procure the S-400," his office said in a statement.

The offer was first made in April but has yet to receive a response.

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

The S-400 purchase would expose Turkey to US sanctions against arms deals with Russia.

Washington says the presence of Russian missile defences in Turkey would be a threat to Western military systems and could jeopardise its involvement in the F-35 fighter jet programme, for which it provides some parts.

But Erdogan has been adamant it is a "done deal", and Defence Minister Hulusi Akar said last week that Turkey had already sent personnel to Russia for training.

On a more positive note, Erdogan told Trump that he welcomed the decision to eliminate tariffs on Turkish steel, saying it would help their countries reach the goal of $75 billion (67 billion euros) in bilateral trade.