More than 12,000 foreigners from 74 countries have gone to fight with rebels in Syria, a leading terrorism expert has said.
Of these, 60-70pc came from other Middle Eastern countries and about 20-25pc from Western nations.
Professor Peter Neumann, who directs the International Centre for the Study of Radicalisation at King's College London, said the Syrian conflict has sparked the most significant mobilisation of foreign fighters since the 1980s war in Afghanistan against the Soviet occupation, where up to 20,000 foreigners participated over the course of a decade.
With more than 12,000 foreigners taking up arms in Syria in just three years, he said, "that conflict is well on track to becoming the most significant mobilisation of foreign fighters that has ever taken place in living memory".
Prof Neumann said that is significant because out of the Afghan conflict came al Qaida and other jihadist networks.
The Syrian conflict is now forging new networks and Prof Neumann said: "I am confident ... that out of that foreign fighter mobilisation, over the course of the next generation there will be terrorist attacks."
Prof Neumann has been consulting the UN Security Council ahead of its September 24 summit meeting, chaired by President Barack Obama, on foreign terrorist fighters and the threat they pose.
He did not give a breakdown on how many foreigners were fighting for the Islamic State (IS) militant group, which has captured a large swath of Syria and Iraq, or for the al Qaida-linked al-Nusra group or other armed groups like the Free Syrian Army.
Prof Neumann said the number of foreign fighters stagnated or decreased from December to June because of infighting between armed groups in Syria, but once the IS group started capturing territory, including Iraq's second-largest city Mosul, and declared a caliphate, it started attracting more foreign fighters.
Tunisia has sent the largest number of foreign fighters to Syria, up to 3,000, he said. Saudi Arabia's government has given two estimates - 1,200 and 2,500 Saudi fighters - while Morocco and Jordan each have about 1,500 though a lot of the Jordanians have tribal connections in Syria, he said.
Among Western countries, there are about 700 foreign fighters from France, more than 500 from Britain, 400 from Germany, 300 from Belgium and 100 from the United States, Prof Neumann said.
"If you take into account per capita population, the most heavily affected countries are the Belgians, the Netherlands and the Scandinavian countries - Denmark, Sweden, Norway - which are small countries but have produced 50-100 fighters each," he said.
Prof Neumann also noted that in some European countries, which he did not identify, between 10-20% of those going to Syria to join IS are women. "The internet has undoubtedly facilitated that because it allows females to participate in a movement that face-to-face it would be completely impossible for them to even be allowed to be in the same room," he said.
Prof Neumann said there are several motivations that attract young men to be foreign fighters - a sense of adventure though the IS group opposed this "because it was attracting the wrong kind of recruit", the claim by the IS and other extremist groups that a genocide was being committed against Sunni Muslims in Syria, and building the caliphate that the Islamic State group has declared.
In the last few weeks, since IS's highly publicised beheadings of two American journalists, Prof Neumann said that "more and more foreign fighters are also talking about fighting against the West and fighting against America".
Prof Neumann said US air strikes in Iraq have stopped the advances and momentum of IS and Kurdish fighters are winning battles where a month ago they were losing battles, which could affect foreign recruitment.