US will withdraw from Syria very soon: Trump

"We're knocking the hell out of IS (Islamic State). We'll be coming out of Syria like very soon. Let the other people take care of it now," Trump told supporters at an event in Ohio on Thursday.

"We are going to have 100 per cent of the caliphate, as they call it, sometimes referred to as land... But we are going to be coming out of there real soon. We are going to get back to our country, where we belong, where we want to be," Trump added.

A US defence official told CNN that it was unclear what the President meant by the comments. 

Trump's remarks on pulling out of Syria were made just hours after Chief Pentagon spokesperson Dana White told reporters that "important work remains to guarantee the lasting defeat of these violent extremists" in the country.

Despite Trump's focus on the terror group's dwindling control of territory, White said the military was focused on more than just IS-controlled areas, saying "our commitment to win must outlast the so-called physical caliphate, and the warped ideas that guide the calculated cruelty of IS".

"We must not relent on IS or permit these terrorists to recover from their battlefield loses," she said, noting that the group still poses a threat.

The official also pointed to several challenges, including figuring out what to do with the some 400 foreign IS fighters currently being held by the Washington-backed Syrian Democratic Forces as one example of the obstacles to a US withdrawal, CNN reported.

He added that the US still needed to decide on a policy with regard to the future of Syrian President Bashar al Assad and Russia's continued military presence in the county as additional complicating factors.