BEIRUT, Lebanon — The commander of the American-backed militia in Syria said Tuesday that it would attack Turkish forces if they enter northeastern Syria, while Turkey’s president, Recep Tayyip Erdogan, indicated that such an operation was imminent.
“We will resist,” Mazlum Kobani, commander of the Kurdish-led militia, said in an interview with The New York Times. “We have been at war for seven years, so we can continue the war for seven more years.”
Mr. Erdogan, speaking to reporters on a flight to Serbia, said the operation might happen before the news could be printed. Turkish troops were being bused to the Syrian border in preparation for an incursion, Turkish media reported. And the Turkish Defense Ministry said on Twitter that preparations to enter Syria “had been completed.”
The escalating challenge came after President Trump agreed to let the Turkish operation go forward and to move American troops out of the way. On Monday, American troops withdrew from posts near two Syrian towns near the border.
The threat of armed resistance from the militia, a force trained and armed by the United States, raises the risks for Turkey as it weighs sending troops into Syria, and for the United States, which could find itself on the sidelines of a new front in Syria’s war — this time between two of its allies.
The militia, the Syrian Democratic Forces, or S.D.F., partnered with the United States to defeat the Islamic State in Syria. Since then, the militia, with American backing, has retained control of a large swath of northeastern Syria.
Turkey considers the militia part of a Kurdish guerrilla movement that threatens Turkey, and Mr. Erdogan has demanded a 20-mile-deep buffer zone along the border that Turkey would control to keep back any Kurdish forces.
Speaking by telephone from Syria, Mr. Kobani said he had been frustrated by the White House’s announcement on Sunday that the United States would stand aside for a Turkish incursion, and that the lack of clear, predictable policies from Washington had made it hard to plan.
“There should not be any ambiguity,” he said.
He spoke of United States troops who had helped his forces fight the Islamic State as comrades-in-arms and said any rupture in that partnership could destabilize the region.
“We fought with U.S. forces to get rid of terrorism, and we are still in this continuing battle,” he said.
He called on Americans to “put pressure on their political and military leaders to stop the Turkish attack,” which he said would lead to “big massacres.”
There was still confusion on Tuesday about Mr. Trump’s new policy.
Mr. Trump said Sunday that the United States would not block a Turkish advance. But on Monday he said that he would “obliterate” Turkey’s economy if its military did anything “off limits,” without defining what that meant, and his aides insisted that he had not given a green light to an invasion.
On Tuesday, he said that he had invited Mr. Erdogan to visit the White House next month.
Two American officials, speaking on condition of anonymity to discuss private diplomatic and military conversations, said that given the apparently contradictory statements by Mr. Trump, the Turks seemed flummoxed about what support, if any, they might get from the United States. As a result, they may be rethinking what to do next, the officials said.
Turkish news media reported that Turkey’s armed forces were preparing F16 jets and Howitzers. Special forces troops were arriving in buses at the border crossing of Akcakale just across from the Syrian town of Tel Abyad, and cranes were moving into position to lift concrete barriers at the border.
Tel Abyad was one of the towns evacuated by American forces on Monday. The other was Ras al Ain. American officials said Tuesday that Turkey had amassed several hundred troops, including tanks and other armor, near the two towns.
Political analysts with knowledge of the plan worked out with American officials said Turkey planned to set up four bases or combat posts in a narrow area along the border, and had agreed to stick to a limited action as a first stage.
“I would expect Turkey to implement a graduated incursion, then go back to negotiation with the U.S. from a stronger position,” said Ozgur Unluhisarcikli, Ankara director of the German Marshall Fund of the United States. “Then when it is in a better situation, do a second operation, and a third, that is a graduated strategy.”
Mr. Trump’s argument that pulling United States forces from Syria was a fulfillment of his vow to get Americans out of “endless wars” unleashed a wave of criticism, much of it from Republican lawmakers. Many argued that withdrawing the roughly 1,000 United States troops in northeastern Syria would open a void that could be exploited by President Bashar al-Assad of Syria or his Russian and Iranian allies, or by the Islamic State.
Mr. Trump has not ordered a full withdrawal from Syria. The order on Sunday was only to relocate roughly 100 to 150 troops that had been stationed near the Turkish border. About two dozen were pulled back on Monday.
But analysts feared that Kurdish troops could be redeployed to the northern border against Turkey, taking them away from the battle against the Islamic State. The Islamic State was driven from its last territory in Syria in February, but the S.D.F., with the support of American Special Operations Forces, continue to battle the group’s remnants.
The American officials said the S.D.F. was already beginning to move off some of its counterterrorism missions against the Islamic State.
“The danger of ISIS is real,” Mr. Kobani said, adding that it maintains sleeper cells throughout the territory. His forces also oversee prisons and camps holding tens of thousands of former Islamic State fighters and their families, which Mr. Trump has said Turkey could take over.
Mr. Kobani said there had been no conversations with the United States about handing over these prisoners to Turkey and he called the idea “impossible.”
If Mr. Kobani had his way, the United States would remain in Syria until the Islamic State and its remnants are destroyed and the country reached a “complete political solution that guarantees everyone’s rights,” he said.
Mr. Kobani, who is in his early 50s, is a lighting rod in the region. United States officials who know him praise his work ethic and professionalism, while the Turks see him as a wolf-in-sheep’s clothing who preaches democracy while plotting attacks on Turkey.
He is an ethnic Kurd from Kobani, a Syrian town on the Turkish border, who attended university in Aleppo, Syria.
By his own admission, he was active in the P.K.K., a Kurdish guerrilla movement that has been fighting the Turkish state for decades, although he avoids detailing his activities. The United States and Turkey consider the P.K.K. a terrorist organization.
Officials in Iraq and Turkey who have tracked his career say he spent time in Europe and in northern Iraq and headed a P.K.K. special forces unit involved in attacks on Turkish troops.
Mr. Kobani says he left the P.K.K. to return to Syria after the civil war broke out in 2011 to organize forces to protect Kurdish communities. That led to his partnership with the United States.
Analysts and officials who know him wonder which part of his background will guide his future — his experience as an American partner promoting self-governance in war-torn Syria or his history as an underground guerrilla operative.
Mr. Kobani says he would prefer to stick with the United States and work for a stable Syria, but that his forces are ready to attack if Turkey invaded.
“There will be lots of resistance if they cross the border,” he said. “We will not accept them on our land in any way.”
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