The United States has halted delivery of equipment related to the stealthy F-35 fighter aircraft to Turkey, sources familiar with the situation said, marking the first concrete U.S. step to block delivery of the jet to the NATO ally in light of Ankara’s planned purchase of a Russian missile defense system.

In recent days, U.S. officials told their Turkish counterparts they will not receive further shipments of F-35 related equipment needed to prepare for the arrival of the stealthy jet, two sources familiar with the situation told Reuters. The aircraft is built by Lockheed Martin Corp.

The sources, speaking on condition of anonymity, said the next shipment of training equipment, and all subsequent shipments of F-35 related material, have been canceled.

Turkish President Tayyip Erdogan has refused to back down from Ankara’s planned purchase of a Russian S-400 missile defense system that the United States has said would compromise the security of F-35 aircraft. Turkey has said it will take delivery of the S-400s in July.

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 United States extradite Islamic cleric Fethullah Gulen, differences over Middle East policy and the war in Syria, and sanctions on Iran.

A Pentagon official had told Reuters in March that the United States had a number of items it could withhold in order to send Turkey a signal that the United States was serious about Ankara dropping its ambition to own the S-400.

The Pentagon did not immediately respond to a request for comment. Turkish officials in Ankara were not immediately available for comment.

The U.S. decision on the F-35s was expected to complicate Turkish Foreign Minister Mevlut Cavusoglu’s planned visit to Washington this week for a NATO summit. The latest development in the F-35 dispute came a day after Erdogan suffered one of his biggest electoral losses in decades in local elections.

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