Acting US defence secretary Patrick Shanahan said he would consult with allies at a meeting of NATO defence ministers in Brussels about the "potential" for an observer force in northeast Syria after American forces pull out.

US-backed forces are currently in the fifth day of a fierce battle to expel IS fighters from their last holdout in eastern Syria -- the final remaining scrap of the "caliphate" the jihadists declared in 2014.

But the US is set to withdraw its 2,000 troops from the country, as announced by President Donald Trump in December, in a move that shocked America's allies and raised security fears for the region.

Asked if Britain would support an observer force with boots on the ground, Williamson did not demur but pledged to carry on the fight against IS, also known as Daesh.

"We recognise the fact the threat of Daesh is going to evolve and it's going to change and it's going to disperse," Williamson said as he arrived for the NATO meeting.

"We will continue to do all that is required to ensure that Britain and our allies remain safe."

Shanahan visited Baghdad on Tuesday to reassure Iraqi leaders after President Donald Trump angered many by saying he wanted to maintain some troops at the Al-Asad airbase, northwest of Baghdad, to keep an eye on Iran.

Afterwards Shanahan said he would use the NATO meeting to discuss "where we can take advantage of the opportunities there... in terms of the potential in northeast Syria to establish an observer force" to ensure stability in the longer term.

The international anti-IS coalition, which includes the US and many NATO countries as well as Middle Eastern nations, could be an option for the proposed force, Shanahan said.