In an interview with CNN, Esper said the US forces were pulling out in phases, and had now moved from the "immediate zone of attack" to a northeastern corridor out of the country.

"Then we'll have another phase that will draw all the forces out. We will temporarily position in Iraq before bringing the troops home. They will be coming home," he said.

Dozens of US armored vehicles arrived Monday at a base in Iraqi Kurdistan.

Asked about US President Donald Trump's comment Monday that a small number of soldiers were being deployed to secure oilfields in Syria, Esper said that was still under discussion.

"But that needs to be worked out in time. The president hasn't approved that yet," he said.

"With regard to deployment, my aim is to keep my options open -- really, the president's options open -- so if events change on the ground in Syria or other parts that we have the flexibility to respond to the president's position," he added.

Esper echoed Trump in defending the abandonment of the Kurdish fighters who bore the brunt of the US-led campaign against the Islamic State (IS) group in Syria.

They were forced to flee from cities they controlled in Syria in the face of a Turkish offensive against them.

He said Washington opposed the Turkish invasion, but: "We are not going to war against a NATO ally and certainly not... with regard to a border that we didn't sign up to defend in the first place."

The United States currently has 5,200 troops in Iraq, part of a US-led international coalition against IS.

Their presence at several bases in Iraq has been the subject of debate there, with pro-Iranian Shiite militias and politicians making frequent demands for their withdrawal.