The Islamic State group is recruiting foreign jihadists on an "unprecedented scale" despite international efforts to stem the tide, according to experts and extracts of a UN report published by Britain's Guardian newspaper.
Latest US figures show that around 1,000 foreign fighters are flocking to fight in Iraq and Syria every month, and experts warn that the newest militants may be more extreme than early recruits.
The number of jihadists travelling to fight since 2010 exceeds the cumulative total of those joining other global extremist organisations over the 20 preceding years by "many times", the UN Security Council study said, according to the Guardian.
"Many foreign fighters that originally left for Syria really did think they were going out for a humanitarian cause," said Erin Marie Saltman, senior researcher at counter-extremism think tank Quilliam.
"Now the stakes are slightly higher. Anyone going over as a foreign fighter now, you have to have been radicalised into believing in martyrdom, so most of those individuals will not actually be expecting to come back," she said.
Russian fighters constitute the biggest single fighting force from a non-Muslim country, numbering over 800, and the US-led air strikes will only strengthen their resolve, according to a local expert.
"They are idealists, fanatics, who believe in a global caliphate as we believed in communism," said Alexei Malashenko, from the Carnegie Endowment in Moscow.
Malashenko said the air campaign against IS jihadists had failed to put off new recruits.
"Air strikes have had no effect on recruitment," he said.
The Central Intelligence Agency estimates there are around 15,000 foreigners fighting with the Islamic State (IS) and other hardcore militant groups, although Saltman suggested the number may be closer to 16,000.
Previous figures showed there were 7,000 foreign jihadists fighting in March, and 12,000 in July suggesting 1,000 a month increase, despite the launch of air strikes against IS combatants three months ago.
"It's safe to say this conflict stands out with the highest rate in the last decade," a US security official told AFP. "All of these numbers are trending upward."
According to the Soufan Group think-tank, the highest numbers of foreign jihadists were from Muslim countries, including 3,000 from Tunisia and 2,500 from Saudi Arabia.
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- IS reach spreading -
If anything, the air strikes in Iraq and Syria could be used as a propaganda tool by IS leaders to attract more recruits, experts warned.
"Any message they can send saying, 'Look at what the West does to Muslims', they will use that as a rallying call," said Simon Palombi, terrorism expert at think-tank Chatham House.
"ISIS have been very savvy when it comes to their propaganda and recruitment, as the 15,000 that have been recruited demonstrates," he said, using an alternative name for the IS group.
The report was produced by a UN committee that monitors al-Qaeda, and concluded that the once mighty and feared extremist group was now "maneuvering for relevance" following the rise of the even more militant IS group, which was booted out of al-Qaeda by leader Ayman al-Zawahiri.
The UN agreed with the US government that "core al-Qaeda remains weak", but argued that its demise had only paved the way for more bloody groups.
There is also growing evidence that IS cells are strengthening across the Middle East and North Africa, with 3,000 fighters already based in Libya, according to Romain Caillet, jihadist expert with French research group IFPO.
- 'Call of Duty' ads -
The UN report put the IS group's recruitment success down to its "cosmopolitan embrace" of modern media and social networking.
"Some of their adverts have pretty much copied 'Call of Duty' (computer game) to recruit that sort of age group, they're looking at young impressionable men," said Palombi.
But the reality of warfare could see the tactic backfire, said the experts.
Some recruits are trying to return home "because of disillusionment, because they have witnessed horrific events," said Saltman.
The UN warned that more nations than ever face the problem of dealing with fighters returning from the battle zone, with figures showing fighters from 81 countries.
To lower the risk of attacks in the home country, returning fighters must be put through deradicalisation programmes, Saltman stressed, and security forces will need to utilise all of their powers.