TOKYO — U.S. Defense Secretary Mark Esper said Tuesday that the U.S. intends to prevent any unilateral invasion by Turkey into northern Syria, saying any such move by the Turks would be unacceptable.

Turkish President Recep Tayyip Erdogan has threatened an imminent attack in the northeast to push back U.S.-allied Syrian Kurdish Forces. American and Turkish military officials have been meeting in Ankara to try and negotiate a settlement to avoid the invasion, and Esper said Tuesday that he believes they have made progress on some of the key issues.

U.S. officials have made clear that an invasion is an extremely risky venture that could threaten the safety of U.S. forces working with the SDF and potentially impede the continued defeat of Islamic State militants in the region.

"What we're going to do is prevent unilateral incursions that would upset, again, these mutual interests that the United States, Turkey and the SDF share with regard to northern Syria," Esper told reporters traveling with him to Japan. He said the U.S. is trying to work out an arrangement that addresses Turkey's concerns, adding, "I'm hopeful we'll get there."

He did not provide details on where progress is being made.

Turkey's President Recep Tayyip Erdogan addresses his ruling party members in Ankara on July 26, 2019.

The dispute further weakens U.S. relations with NATO ally Turkey, coming closely on the heels of the Trump administration's decision to remove Ankara from the American-led F-35 fighter aircraft program because it is buying a Russian air defense system that would aid Moscow's intelligence.

The U.S. government’s concern is that the Russian S-400 system could be used to gather data on the capabilities of the F-35, and that the information could end up in Russian hands.

× Fear of missing out? Sign up for the Early Bird Brief - a daily roundup of military and defense news stories from around the globe. Thanks for signing up. By giving us your email, you are opting in to the Early Bird Brief.

Esper said the U.S. will not abandon its SDF allies.

Hundreds of U.S. troops are stationed east of the Euphrates River in northern Syria working with the SDF, and an incursion by Turkey could put them in the middle of any firefight between Turkish and Kurdish forces.

Syrian Kurdish fighters from the People's Protection Units, or YPG, are the key element of the SDF. Turkey considers the YPG an existential threat and as terrorists with close links to a decades-long insurgency within its own border led by the Kurdistan Workers' Party, or PKK.

The SDF, however, has been America's main partner on the ground against the Islamic State group, and the Kurds are currently detaining thousands of foreign fighters. U.S. officials worry that those fighters could get free during a Turkish invasion into the Kurdish-held territory.

Turkey and the U.S. have been negotiating for months over the establishment of a safe zone along the Syrian border that would extend east of the Euphrates to Iraq.

Turkey wants to establish a 25-mile-deep zone. But so far the two sides have failed to reach an agreement.

Esper said Turkey is a long-standing ally and the U.S. is taking this one day at a time. And he suggested that the SDF issue is not new and is markedly different than an ally buying a Russian-made air defense system that could threaten an American aircraft.

“We’ve all seen this before. They have long-standing concerns about the PKK,” said Esper. “That’s why we want to work with them to address their legitimate security concerns going forward.”