A year-long bombing campaign in Syria has had virtually no impact on ISIS numbers, which remain consistently between 20,000 and 30,000, according to the CIA, Daily Mail reports.

The U.S. agency claims the bombings, which have cost billions, have prevented the "collapse of Iraq", but the organisation is fundamentally no weaker than it was before.

It's believed more than 10,000 people have been killed but the organisation is able to constantly replenish its ranks with foreign fighters.

Numbers are still estimated to be between 20,000 and 30,000, the same as last August when the U.S.-led strikes began.

They also remain well-funded, with some estimating the group clears up to £320million a year in oil sales and has seized up to $1billion from banks in its territories.

The CIA also concluded that it could take more than a decade to drive ISIS out of strongholds, according to intelligence analysts.

But the military campaign has put ISIS under increasing pressure in northern Syria, particularly squeezing its self-proclaimed capital in Raqqa.

ISIS remains a well-funded extremist army able to replenish its ranks with foreign terrorists as quickly as the US can eliminate them.

Meanwhile, the group has expanded to other countries, including Libya, Egypt's Sinai Peninsula and Afghanistan.

The assessments by the CIA, the Defence Intelligence Agency and others appear to contradict the optimistic line taken by the Obama administration's special envoy, retired general John Allen, who told a forum in Aspen, Colorado, last week that 'ISIS is losing' in Iraq and Syria.

ISIS's staying power also raises questions about the US administration's approach to combat terrorism.

Officials do not believe it is planning complex attacks on the West from its territory, the group's call to Western Muslims to kill at home has become a serious problem, FBI director James Comey and others say.