A day after US halted F-35 delivery, the acting US defence secretary says he expects to resolve row over Russia’s S-400.

Acting US Secretary of Defense Patrick Shanahan said on Tuesday that he expected to resolve a dispute with Turkey over its purchase of Russia’s S-400 air defence system, a day after the United States halted the delivery of equipment related to the F-35 aircraft to Ankara.

The US is at an inflexion point in a years-long standoff with Turkey, a NATO ally, after failing to sway President Tayyip Erdogan that buying a Russian air defence system would compromise the security of F-35 aircraft.

On Monday, the Pentagon said it had suspended the delivery of equipment related to the stealthy F-35 fighter jet “pending an unequivocal Turkish decision to forgo delivery of the S-400”.

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 defences.

Unusual NATO member move

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

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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, Sergey Lavrov, and insisted that the S-400 purchase would go ahead.

But on Monday, Shanahan expressed optimism that the US and Turkey would find a way out of the crisis by persuading Turkey to buy the Patriot defence system, instead of S-400s.

“I expect we’ll solve the problem so that they have the right defence equipment in terms of Patriots and F-35s,” Shanahan told reporters at the Pentagon.

Shanahan added that he expected the US to ultimately carry out the delivery of F-35s currently at Luke Air Force base to Turkey, after resolving the dispute. Turkish pilots are receiving training on two aircraft at Luke Air Force Base in Arizona.

The disagreement over the F-35 is the latest of a series of diplomatic disputes between the United States and Turkey including Turkish demands that the US extradite preacher and businessman Fethullah Gulen, differences over Middle East policy and the war in Syria.