TOKYO — Defense Secretary Mark Esper Mark EsperOvernight Defense: Stopgap spending measure awaits Senate vote | Trump nominates former Nunes aide for intelligence community watchdog | Trump extends ban on racial discrimination training to contractors, military Overnight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Official: Pentagon has started 'prudent planning' for full Afghanistan withdrawal by May MORE said Tuesday that the United States would find a Turkish incursion into northern Syria “unacceptable” and would seek to prevent such an operation.

“Clearly, we believe any unilateral action by them [Turkey] would be unacceptable,” Esper told reporters traveling with him to Japan.

Turkish President Recep Tayyip Erdoğan on Sunday repeated a threat to cross into Syria, going against U.S. wishes, if Turkey’s conditions for a safe zone in the northern part of the country are not met. Turkey wants such a safe zone to be free of the Kurdish-led Syrian Democratic Forces (SDF), which Washington has partnered with to defeat ISIS fighters but Ankara considers a terrorist group.

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“We can only be patient for so long,” Erdoğan said.

Esper said on Tuesday the United States will “prevent unilateral incursions that would upset these mutual interests that ... the United States, Turkey and the SDF share with regard to Syria.”

He added that the United States currently has a Defense Department team in Turkey to negotiate with Turkish officials on establishing a safe zone for the SDF.

“We’ve made progress on some of the key issues,” Esper said, though he declined to share specifics.

The SDF have expressed fears the U.S. would not come to its aid in the event that Turkey launches an operation to remove them from the Syrian-Turkish border following President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE’s announcement last December that he would seek to pull all U.S. troops from Syria.

But Esper said the Pentagon also continues to talk with the SDF in the region “as much as we do with the Turks,” and has no “ambition to abandon” the U.S.-backed fighters.

Asked what Turkey would risk if it launched an operation into Syria, Esper deferred.

“We have a lot of mutual interests in northern Syria. We want to sustain the continued defeat — at least of the physical caliphate — of ISIS. That becomes a question if [the Turks] move in and the SDF is impacted,” Esper said.

“Again, I’m hopeful we’ll work out something to address their security concerns, we just need to take one day at a time and continue to work through the process.”

Relations between the United States and Turkey have soured in the past several years, most recently over Washington’s move to pull Ankara from the F-35 fighter jet program after the NATO ally took delivery of Russia’s S-400 missile defense system, which is not operable with the advanced fighter jet.