The number of foreigners who have travelled to Iraq and Syria to fight has topped 20,000, surpassing the number attracted to Afghanistan in the 1980s, according to the International Centre for the Study of Radicalisation (ICSR).
Centre director Peter Neumann, who has advised the United Nations Security Council on the foreign fighters issue, said the conflict had become a "truly international" fight.
Counting the number of Western fighters was "no exact science", Professor Neumann said, but his estimates were based on more than 1,500 sources, including media reports, government estimates, social media profiles, statements from jihadist groups, direct interviews and fieldwork.
The 20,000 figure is an estimate of the total number of foreigners who have joined the fight over the course of the conflict, rather than the number currently engaged in Iraq and Syria.
The centre's research suggested up to one-third of foreign fighters, or nearly 7,000, had already returned to their home countries and between 5 per cent and 10 per cent had been killed.
"There has been actually only one mobilisation of foreign fighters that has been similar, and that is of course the Afghanistan conflict in the 1980s, which has also produced up to 20,000 fighters - albeit over an entire decade, whereas in the case of Syria and Iraq we're now talking about the same number in just three or four years," Professor Neumann said in a speech at the London School of Economics.
As with Afghanistan, the current mobilisation would have "long-lasting consequences", he added.
More than half of the foreign fighters who had travelled to Iraq and Syria were from the Middle East, Professor Neumann said.
But recruits from Western Europe had climbed to almost 4,000, with the most populous nations - France, the UK and Germany - producing the largest number of fighters.
However, a number of smaller European countries contributed to the overall totals disproportionately, considering their populations.
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Explained: Where do foreign fighters in Iraq and Syria come from?