"My takeaway is that I felt very welcomed and it was a very inclusive environment," Shanahan said, speaking aboard the airplane bringing him home from a three-day international security conference in Munich.

"I was expecting something different. I was expecting them to be a bit more stand-offish," he said, a reference to criticism from allies over President Donald Trump's unilateral decision to pull some 2,000 US troops from Syria.

"The type of dialog and discourse was very frank. I like those types of meetings," he said.

He said that the Pentagon, in coordination with the State Department, will look at the issues raised by Europeans "so we can resolve these constraints and be able to answer a number of the questions... so we can do more planning."

Shanahan on Friday struggled to convince skeptical allies in the international coalition against the Islamic State (IS) jihadist group to help secure Syria once US soldiers pull out.

As the end neared for IS proto-state that once controlled large areas of Iraq and Syria, 13 defense ministers of the anti-IS coalition met on the sidelines of the Munich Security Conference.

Shanahan pledged ongoing backing for the fight -- but kept allies guessing as to how that would be achieved once US forces pull out, and won no solid pledges of support.

Shanahan pledged that the US would "maintain our counterterrorism capabilities in the region" and "continue to support our local partners' ability to stand up to the remnants of (IS)" -- but gave no details about how this would be done.