Syria has become the pre-eminent global incubator for a new generation of militants after Islamist groups more than doubled the recruitment of foreign fighters to as much as 31,000 over the past 18 months, according to a former British spy chief.
In the chaos of Syria's civil war, the majority of foreign fighters end up in militant groups like Islamic State, which uses an extreme interpretation of Islam to justify attacks on its foes and impose highly repressive rule in large swathes of Syria and neighboring Iraq that it has captured.
"The Islamic State has seen success beyond the dreams of other terrorist groups that now appear conventional and even old-fashioned, such as al-Qaida," said Richard Barrett, who was formerly head of global counter-terrorism at Britain's Secret Intelligence Service, known as MI6.
'More than doubled'
"Despite sustained international effort to contain the Islamic State and stem the flow of militants traveling to Syria, the number of foreign fighters has more than doubled," Barrett said in an emailed report.
Islamic State militants claimed responsibility for a November 13 attack on Paris that killed 130 people and the October 31 downing of a Russian passenger plane over Egypt's Sinai region that killed 224.
They promise more attacks on the West and Russia.
Al-Qaida militants killed nearly 3,000 people in the September 11, 2001, attacks on the United States.
Western leaders say the Islamic State group, which has proclaimed a caliphate across parts of Syria and Iraq, now poses a greater danger.
Attacks claimed by Islamic State have prompted intensified airstrikes on the militants by a U.S.-led coalition that includes Britain and France. Russia, which supports Syrian President Bashar al-Assad, is also bombing Islamic State as well as various anti-Assad rebel groups in Syria.
Western intelligence officials say foreign fighters who join militants in Syria pose a menace to national security as they can become battle-hardened and radicalized in Syria before returning with ease to Western states to carry out attacks.
"Even if the Islamic State is a failing enterprise in steady decline, it will be able to influence the actions of its adherents, and it may become more dangerous as it dies," Barrett wrote.
Barrett, who now works for the Soufan Group, a New York intelligence consultancy, said most of the foreign fighters flocking to Syria are Arabs from the Middle East and Africa.
Significant numbers came from Tunisia, Saudi Arabia and the former Soviet Union, while about 5,000 have traveled from the European Union, Barrett said in the report.
The headline estimate of 27,000-31,000 foreign fighters who have traveled to Syria and Iraq compares to an estimate of 12,000 foreign fighters who had journeyed to Syria that Barrett made in June 2014.
Though individual countries provide estimates of the number of their nationals who have gone to Syria, there are few broad global estimates. Barrett's figures include those who may have gone to Syria and subsequently returned.
Militants in Syria have attracted fighters from across 86 different countries ranging from Norway to Uzbekistan, with steep increases from the Middle East, North Africa and the former Soviet Union.
Around 6,000 fighters from Tunisia have gone to Syria, 2,500 from Saudi Arabia and 2,400 from Russia, according to Barrett.
Of the roughly 5,000 EU recruits, around 3,700 come from four countries - France, Britain, Germany and Belgium.
The figure for France may be overstated as officials say 1,800 citizens and residents are linked to Islamic State but that this number encompasses those who have been killed, are in prison, returned or may be planning to go.
North Caucasus, Central Asia
Recruits from the former Soviet Union, particularly Russia's North Caucasus republics of Chechnya and Dagestan and the Muslim countries of Central Asia, have also dramatically risen.
"As the Islamic State changes its focus from consolidating control of territory to attacking its foreign enemies in their own homelands, or their interests elsewhere, the profile of its foreign recruits will also change," Barrett said.
"The Syrian civil war will not end soon," added Barrett, who between 2004 and 2013 headed the United Nations team that monitored al Qaeda and the Taliban.