A day after Washington put a freeze on its joint F-35 fighter jet program with Turkey in protest to the S-400 deal, Shanahan said he expects Ankara to opt for the Patriot missiles.

That would then allow the F-35 program to continue, he confirmed.

"I've had a number of conversations with Defense Minister (Hulusi) Akar and I really think we'll resolve this situation with our strategic partners," he said.

"I am very confident in the Patriot proposal that we've delivered to Turkey, its availability, it's pricing, and very importantly, the industrial participation that comes along with the Patriot system."

Asked if that meant he expects Turkey will buy the Patriot system, Shanahan replied: "I expect we'll solve the problem so that they have the right defense equipment in terms of Patriots and F-35s."

As for the F-35s planned for Turkey, he said: "I expect them to be delivered."

On Monday, the Pentagon said it was halting all deliveries and joint work with Turkey on the F-35 if Ankara insisted on buying the S-400 system from Russia, a move which has alarmed Turkey's NATO allies.

The tough step came after months of warnings from Washington that Turkey's adoption of Russian missile technology alongside US fighter jets would pose a threat to the F-35 technology and endanger Western defenses.

"Pending an unequivocal Turkish decision to forgo delivery of the S-400, deliveries and activities associated with the stand-up of Turkey's F-35 operational capability have been suspended while our dialogue on this important matter continues with Turkey," the Pentagon said in a statement.

The purchase of a Russian system is highly unusual for a member of the North Atlantic Treaty Organization, the Western alliance forged to counter the Soviet Union.

The US suspension announcement came two days before foreign ministers from NATO's 29 members were due in Washington to celebrate the 70th anniversary of the alliance.

Last week, Foreign Minister Mevlut Cavusoglu met in Turkey with his Russian counterpart, Sergei Lavrov, and insisted that the S-400 purchase would go ahead.