Fighters of the US-backed Syrian Democratic Forces said they had captured the last piece of territory held by the militants on Saturday.

They declared the so-called caliphate which once stretched across swathes of Iraq and Syria to be finished.

"The liberation of the last territory held by ISIS/Daesh is a remarkable achievement of the Global @coalition. #NATO remains committed to our common fight against terrorism," Stoltenberg tweeted.

While NATO has not had a combat role in the fight against IS, it has provided reconnaissance aircraft to the global coalition to defeat the insurgent group.

The state proclaimed in mid-2014 by fugitive IS supremo Abu Bakr al-Baghdadi started collapsing in 2017 when parallel offensives in Iraq and Syria wrested back its main hubs Mosul and Raqa.