Speaking to reporters en route to the Middle East late Saturday night, Esper said the Pentagon was yet to work out on how exactly it would conduct its campaign outside of Syria, reports Efe news.
Among the issues to be addressed were how to conduct the US air mission, the role of allies and whether a small number of US troops could re-enter Syria to execute specific missions against the militant group, the defense chief added.
Esper said one reason for his week-long trip to the Middle East and to the NATO headquarters in Brussels was to iron out the specifics of the campaign against the IS.
President Donald Trump's administration announced a full withdrawal of troops from northeastern Syria shortly before Turkey launched its offensive against the Kurds in the region on October 9.
Trump's decision to move all US troops out of Syria has met with bipartisan disapproval in Congress, where lawmakers charge that the administration abandoned Kurdish fighters who were key to a series of battles that led to the collapse of the IS' self-proclaimed "caliphate".
Esper said on Saturday night added that he would be meeting with allies on a key question: "What does the next phase of the counter-IS campaign look like?", adding that the US hoped to continue working with Kurdish forces.
The US troops moving to Iraq have so far been assigned to conduct the counterterrorism mission against the IS and "help defend Iraq," he said. "That's the game plan right now."
Esper said he spoke to his Iraqi counterpart about the arrival of the troops. There were roughly 5,400 American troops already there before the US began leaving Syria a week ago.
US troops currently are leaving Syria via helicopters, planes and ground convoys, a process that would be completed within weeks, "not days", Esper said. He didn't say where precisely in western Iraq those troops would be based.
Roughly 150 US troops will remain in the southern Syrian city of al-Tanf, a garrison unaffected by Trump's withdrawal order.
On Thursday, Vice President Mike Pence said that Turkey had agreed to a five-day cessation of border hostilities in return for a Kurdish withdrawal from what Erdogan has described as a safe zone along the border.
Turkey considers US-backed Kurdish fighters in that area to be terrorists.
On Saturday, the area saw intermittent fighting amid a tense calm.
Turkey and the Kurds have charged each other with violating the agreement.
The Kurdish-led Syrian Democratic Forces said that since the pact, Turkish strikes have killed 20 civil and 14 of its fighters.
Turkish officials said their troops have complied with the agreement.
( With inputs from IANS )