Speaking at a conference in Washington, British Army Major General Felix Gedney, a deputy commander in the coalition, said that the coalition estimates that fewer than 1,000 IS fighters remain in Iraq and Syria.

Gedney said the fight against IS was not over, and militants, in particular, have been seen moving west of the Euphrates River in areas controlled by Syria and its allies.

He also accused the Syrian government of allowing ISIS gangs to move freely through the territory it controls.

"They seem to be moving with impunity through the regime-held territory, showing that the regime is clearly either unwilling or unable to defeat Daesh within their borders," Gedney said.

"We would call on the Syrian regime to clear ISIS from those areas that are currently under their control" he added.