The rise of ISIS: Group of 10,000 fighters now controls a territory the size of Britain

ISIS now controls territory the size of the UK, is making £600,000 a day from selling oil and ransoming hostages and has a fighting force of 10,000 militants, according to a leading expert.

Matthew Olsen, a senior US counter-terrorism official, said the Islamic State, also known as ISIS, was “an extremely dangerous organisation operating in a chaotic part of the world” and saw itself “as the new leader of the global jihadist movement”.

ISIS began in Syria during the rebellion against President Bashar Assad, but took huge swathes of northern Iraq earlier this year.

Olsen said Isis had expanded quickly partly because of the weakness of the governments of Syria and Iraq.”ISIS threatens to outpace al-Qa’ida as the dominant voice of influence in the global extremist movement,” he said, according to the Daily Telegraph.

But he added: “As formidable as ISIS is as a group, it is not invincible. With a broad coalition of international partners, we have the tools to defeat ISIS based on a determined and comprehensive all-of-government approach.”

Air strikes by US warplanes in combination with attacks by Iraqi and Kurdish troops had showed Isis was “vulnerable to coordinated and effective military action”, he said.”The strikes have begun to sap ISIS’s momentum and created the space for Iraqi and Kurdish forces to take the offensive,” he said.

But Olsen warned there were others who could pose a threat to the West. “ISIS has captured our immediate focus, but it is only one of the myriad groups that pose a threat to us at the terrorist landscape evolves,” he said.