The surge in foreign militants heading to Syria surpasses anything seen in previous conflicts in the past 10 years, including wars in Afghanistan, Yemen, Somalia or elsewhere, said a US intelligence official, who spoke on condition of anonymity.
"It's safe to say this conflict stands out with the highest rate in the last decade," the official told AFP.
US intelligence agencies now believe there are more than 16,000 volunteers from 80 countries that have traveled to Syria to fight with various militant groups, including the Islamic State jihadists who have seized large swathes of territory in Iraq and Syria.
"All of these numbers are trending upward," the official said.
There was no sign yet that US-led air strikes against the Islamic State group in Syria and Iraq or new restrictions aimed at foreign volunteers in several countries had affected the flow, officials said.
Analysts say the numbers may be much higher while US officials acknowledge that tracking the movement of foreign volunteers remains a challenge -- and that any figure released is only a rough estimate.
"We have limited insights on this," the official added.
Most of the volunteers have come from the Middle East and North Africa, with the highest number of fighters flowing out of Tunisia, officials said.
More than 2,000 hail from European nations and more than 100 from the United States, with a small number -- about a dozen -- linked to the Islamic State group, according US estimates.
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- Returning radicalized -
The numbers on foreign fighters were first reported by NBC News and the Washington Post.
James Clapper, director of national intelligence, warned this week the foreign fighters may return to their countries ready to orchestrate violence.
"First and foremost, the more than 16,000 foreign fighters who gravitated to Syria are now returning to their countries of origin, including in the West," Clapper said at an event Monday.
"They've picked up dangerous skills and radicalization, both at the same time," Clapper said.
The chaos in Syria -- where a civil war has raged for three years between the regime of President Bashar al-Assad and opposition forces -- porous borders and the sophisticated use of social media by hardline militants to recruit volunteers have all paved the way for the influx of foreign fighters, according to US officials and experts.
The Islamic State jihadists in particular have exploited social media as a powerful propaganda tool, unlike Al-Qaeda, according to Michael Leiter, former head of the National Counterterrorism Center.
The Islamic State group "is in social media, and it is going after 'Jihadi Cool,'" Leiter told NBC's "Meet the Press" on Sunday.
"What we have to do now is counter that message using social media just as effectively. And that's not something the US government over the past 10 years has been particularly good at," he said.