Jihadists from more than 80 countries have flocked to fight in Iraq and Syria on an "unprecedented scale", according to extracts of a UN report published by Britain's Guardian newspaper on Friday.
Around 15,000 people have travelled to fight alongside Islamic State (IS) and other hardcore militant groups from "countries that have not previously faced challenges relating to al-Qaeda," said the report.
The number of foreign jihadists travelling to fight since 2010 exceeds the cumulative total of the 20 preceding years "many times", added the Security Council study.
"There are instances of foreign terrorist fighters from France, the Russian Federation and the United Kingdom of Great Britain and Northern Ireland operating together," it said, according to the Guardian.
Britain's top police officer, Bernard Hogan-Howe, estimated last week that five people a week were leaving the country to fight with IS. Security officials estimate that there are currently around 500 British nationals fighting in Syria and Iraq.
The UN warned that more nations than ever face the problem of dealing with fighters returning from the battle zone.
The report was produced by a committee that monitors al-Qaeda, and concluded that the once mighty and feared group was now "maneuvering for relevance" following the rise of the even more militant IS, which was booted out of al-Qaeda by leader Ayman al-Zawahiri.
Despite the split, the UN concluded that the legal basis for US President Barack Obama's fight against IS was justified by its ideological congruence with al-Qaeda, and considered the two groups as part of a broader movement.
"Al-Qaeda core and Isil (IS) pursue similar strategic goals, albeit with tactical differences regarding sequencing and substantive differences about personal leadership," the UN wrote.
Obama has vowed he will not order a large force into combat in Iraq or Syria, relying instead on air power and local forces.
But his "no boots on the ground" pledge is coming under pressure amid growing calls for advisers and forward air controllers to deploy with Iraqi or Kurdish soldiers to help direct air raids and plan operations.
The IS group's "cosmopolitan" use of social media, "as when extremists post kitten photographs", was attracting a new breed of foreign fighters who are put off by the more dogmatic communication tactics of al-Qaeda, said the report.
IS leaders recognise "the terror and recruitment value of multichannel, multi-language social and other media messaging," it added.
The UN agreed with the Obama administration that "core al-Qaeda remains weak", but argued that its demise had only paved the way for more bloody groups, for whom "cross-border attacks – or attacks against international targets – remain a minority."