Islamic State still has as many as 30,000 members in Syria and Iraq – a far higher number than previously estimated, a United Nations report has found.
The report by UN sanctions monitors said of those 20,000-30,000, which are equally distributed between the two countries, several thousand were thought to be foreign fighters.
The US-led coalition against Islamic State of Iraq and the Levant (Isil) claimed last December that there were fewer than 1,000 fighters remaining in Iraq and Syria and that the group was close to being defeated.
At the height of its reign in 2015 Isil counted as many as 100,000 jihadist in its ranks, controlling territory spanning Syria and neighbouring Iraq that was roughly the size of the UK.
Its members are now confined to a sliver of land between the Syria-Iraq border and maintain control of just small pockets of territory in central and southern Syria, having been pushed back by US-backed local forces.
With its physical caliphate largely destroyed, the Islamic State movement is transforming from a "proto-state" to a covert "terrorist" network, "a process that is most advanced in Iraq", the report said.
“Despite the damage to bureaucratic structures of the so-called ‘caliphate,’ the collective discipline of Isil is intact,” it continued. “Although he is reported to have been injured, Abu Bakr al-Baghdadi remains in authority.”
In Iraq, where Baghdadi pronounced the caliphate, jihadist sleeper cells have plagued areas such as Kirkuk and Salahaddin with insurgent attacks.
The UN believes thousands are hiding out in Iraq among sympathetic communities as well as disguised in larger, more urban areas.
Joel Wing, an Iraq analyst who has been monitoring attacks by Isil, recorded more than 100 IED explosions and more than 80 shootings in the north of the country since the beginning of the year.
In Syria, the group continues to be able to mount ambitious attacks, including one on the southern city of Sweida which is thought to be its deadliest in recent years.
Some 250 people, largely from the minority Druze community, were killed in coordinated suicide bombings and shootings across the city.
Some UN member-states also raised concerns with the authors of the report that new Isil cells were emerging from the densely populated Rukban camp for refugees in southern Syria, on the border with Jordan, where families of Isil fighters are now living.
Thomas Joscelyn, a senior fellow at the Foundation for Defence of Democracies, believes the coalition wildly underestimated the number of fighters Isil had at its height, and therefore its calculations of how many remained were too low.
Mr Joscelyn told The Telegraph that some of those still fighting will be new recruits, “however, the Islamic State likely retains a hardened cadre of fighters who have survived the group’s many battles,” he said.
To date, the US has spent $14.3 billion (£11.2 billion) on more than 24,000 airstrikes to destroy Isil in Iraq Syria, killing some 50,000 jihadists in the battlefield.
In March, President Donald Trump ordered a withdrawal of the US’s 2,200 troops in Syria, however experts have warned such a move would be premature.
The report said that a “covert” Isil core will continue to exist in both countries, with its ideology much more difficult to snuff out.
“There is that danger, especially as Isil is conducting daily attacks in Iraq and Syria,” said Mr Jocelyn. “The group hasn’t been defeated. Isil knew it was going to lose its territorial caliphate and its leaders took steps to continue fighting as insurgents.”