Speaking to reporters traveling with him to the Middle East, Esper did not rule out the idea that U.S. forces would conduct counterterrorism missions from Iraq into Syria. But he said those details will be worked out over time.
His comments were the first to specifically lay out where American troops will go as they leave Syria and what the counter-IS fight could look like. Esper said he has spoken to his Iraqi counterpart about the plan to shift the more than 700 troops leaving Syria into western Iraq.
As he left Washington Saturday, U.S. military troops were continuing to pull out of northern Syria in the wake of Turkey's invasion into the border region. Reports of sporadic clashes continued between Turkish-backed fighters and the U.S.-allied Syria Kurdish forces despite a five-day cease-fire agreement hammered out on Friday between U.S. and Turkish leaders.
Trump ordered the bulk of the approximately 1,000 U.S. troops in Syria to withdraw after Turkish President Recep Tayyip Erdogan made it clear in a phone call that his forces were about to invade Syria to push back Kurdish forces that Turkey considers terrorists.
The pullout largely abandons the Kurdish allies who have fought the Islamic State group alongside U.S. troops for several years. Between 200 and 300 U.S. troops will remain at the southern Syrian outpost of Al-Tanf.
Esper said the troops going into Iraq will have two missions, including countering any threat from IS in Iraq.
"One is to help defend Iraq and two is to perform a counter-ISIS mission as we sort through the next steps," he said. "Things could change between now and whenever we complete the withdrawal, but that's the game plan right now."
The U.S. currently has more than 5,000 American forces in Iraq, under an agreement between the two countries. The U.S. pulled its troops out of Iraq in 2011 when combat operations there ended, but they went back in after the Islamic State group began to take over large swaths of the country in 2014. The number of American forces in Iraq has remained small due to political sensitivities in the country, after years of what some Iraqis consider U.S. occupation during the war that began in 2003.
Esper said he will meet with other allies at a NATO meeting next week to discuss the way ahead for the counter-IS mission.
Asked if U.S. special operations forces will conduct unilateral military operations into Syria to go after IS, Esper said that is an option that will be discussed with allies over time.
He said one of his top concerns is what the next phase of the counter-IS missions looks like, "but we have to work through those details. He said that if U.S. forces do go in, they would be protected by American aircraft.
While he acknowledged reports of intermittent fighting despite the cease-fire agreement, he said that overall it "generally seems to be holding. We see a stability of the lines, if you will, on the ground."
He also said that, so far, the Syrian Democratic Forces that partnered with the U.S. to fight IS have maintained control of the prisons in Syria where they are still present. The Turks, he said, have indicated they have control of the IS prisons in their areas.
"I can't assess whether that's true or not without having people on the ground," said Esper.
He added that the U.S. withdrawal will be deliberate and safe, and will take "weeks not days."
According to a U.S. official on Saturday, about a couple hundred troops have left Syria so far. The U.S. forces have been largely consolidated in one location in the west and a few locations in the east.
The official, who spoke on condition of anonymity to discuss ongoing operations, said the U.S. military is not closely monitoring the effectiveness of the ceasefire, but is aware of sporadic fighting and violations of the agreement. The official said it will still take a couple weeks to get forces out of Syria.