Mark Esper, nominated as defense secretary last Friday, reiterated Washington's warning that buying the Russian S-400 missile system would mean it woud be booted out of the US F-35 jet fighter programme.

Making his first appearance at a meeting of NATO defence ministers, Esper had what a Pentagon spokesman described as a "frank and transparent" meeting with his Turkish counterpart Hulusi Akar.

The US and NATO are alarmed that Turkey may acquire the missiles, which are designed to shoot down planes like the F-35, America's new generation multi-role stealth fighter that Turkey also wants to buy.

But Turkey has so far rebuffed all efforts to persuade it to drop the deal, and President Recep Tayyip Erdogan insisted on Tuesday that delivery would take place in July.

"The two leaders had a frank and transparent discussion where Secretary Esper reiterated that Turkey's purchase of the Russian S-400 air and missile defense system is incompatible with the F-35 program and that Turkey will not be permitted to have both systems," Pentagon spokesman Jonathan Hoffman said.

Erdogan has said he will use his good relations with Trump to defuse tensions when they meet on the sidelines of the G20 summit in Osaka, Japan later this week.

But the US has set a July 31 deadline for Ankara to drop the purchase or face sanctions and expulsion from the F-35 programme.

This would mean an end to Turkey's current plans to buy 100 F-35s and the loss of lucrative contracts to build parts of the jet.

Esper, who is yet to be confirmed in post by the Senate, is the third man to lead the Pentagon in six months after Jim Mattis and Patrick Shanahan.

The departure of defense secretary Mattis -- who quit in December admitting he had had differences with Trump -- concerned some European allies who saw him as a cool, experienced head in Washington.

In Esper they face another former military man, but one who is close to Secretary of State Mike Pompeo and National Security Advisor John Bolton -- both outspoken hawks in the Trump team.