Secretary of Defense Mark Esper and Chairman of the Joint Chiefs of Staff Gen. Joe Dunford address America's relationship with Turkey during a Pentagon briefing.

WASHINGTON — The Pentagon would consider allowing Turkey to rejoin the F-35 program only if the Russian-made S-400 air defense system is completely removed from Turkish soil, meaning the government in Ankara could not simply keep the systems deactivated in warehouses, the Pentagon’s top official said Wednesday.

“They have to, again, get rid of the S-400 program and completely out of the country [before] we could consider that,” Secretary of Defense Mark Esper told press when asked about the possibility Turkey could find a way to again be part of the Joint Strike Fighter program.

“I have been very clear in both my public comments and privately with my Turkish counterpart: It’s either the F-35 or the S-400,” he said. “It’s not both. It’s not park one in the garage and roll the other one out. It’s one or the other. So we are where we are, and it’s regrettable.”

Turkey, a partner in the F-35 program that helped fund the development of the jet, planned to buy 100 F-35As. However, a decision by Ankara to purchase the S-400 threw a political bomb into the F-35 acquisition plan; the U.S. and its NATO allies expressed major concerns about the Russian system sharing airspace with the alliance’s newest fighter.

In July, Turkey took possession of the first S-400 pieces, and the U.S. formally kicked the country out of the F-35 program. As a result, by March 2020, Turkey’s industrial participation in the F-35 program will be done, with work shifted to U.S. companies.

The secretary’s comments were the first official confirmation that a path, however narrow, for Turkey to rejoin the program does exist.

Some analysts have questioned whether a loophole exists that would allow Ankara to back down, save face and regain the fifth-generation fighter. Should Turkey keep the S-400 parts in a warehouse somewhere not actively running, the argument goes, the U.S. could invite Turkey back to the F-35 program.

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But Esper seemed to shut that down, saying: “No, not in my book,” when asked about the possibility.

Appearing with Esper was Chairman of the Joint Chiefs of Staff Gen. Joe Dunford, who expounded on the overall relationship with Turkey.

“When I look at Turkey and the United States, it’s very clear to me that we have many more areas of convergence than divergence,” said Dunford, who has spent significant time traveling to Turkey since the failed coup of 2016. “And many of these areas of divergence are kind of near-term issues. They are difficult issues, no question about it, but they’re issues that we can work through. So we try to focus on the horizon.