The head of the Pentagon cautioned Turkey on Tuesday against undertaking a unilateral military operation in northern Syria to target a US-allied Kurdish militia as American defense officials visited Ankara in what was seen as an eleventh-hour effort by Washington to block a Turkish attack.

Tensions between the NATO allies over the border region that Turkey considers part of its national security sphere have come to a head as President Recep Tayyip Erdogan warns he is running out of patience with the long-running negotiations. The two sides are discussing the size and scope of a buffer zone south of the Turkish border that would expel the Syrian Democratic Forces, led by Kurdish militants that Ankara considers a terrorist organization.

The United States will prevent a unilateral incursion that would harm the two sides’ “mutual interests” in Syria, US Defense Secretary Mark Esper told reporters as he traveled to Japan. "Clearly we believe any unilateral action by them would be unacceptable," he said. “What we’re trying to do now is work out an arrangement to address their concerns, and I’m hopeful we will get there.”

Erdogan said on Tuesday that “drying out the terrorist swamp” in northern Syria was his country’s top priority and that it would build on two previous cross-border operations, in 2016 and 2018, that broke up a swathe of territory under Kurdish control.