Syria has become the pre-eminent global incubator for a new generation of militants after Islamist groups more than doubled their 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 repressive rule in the 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 Qaeda," said Richard Barrett, a former head of global counter-terrorism at Britain's Secret Intelligence Service, known as MI6.

"The number of foreigners joining the so-called Islamic State in Syria and Iraq has doubled, despite the effort to prevent them doing so," Barrett, now senior vice president of the Soufan Group, a New York intelligence consultancy, wrote in a report.

Islamic State militants claimed responsibility for the Nov. 13 attacks in Paris that killed 130 people and the Oct. 31 downing of a Russian passenger plane over Egypt's Sinai region that killed 224. They have promised more attacks on the West and Russia.

Al Qaeda militants killed nearly 3,000 people in the Sept. 11, 2001 attacks on the United States.

Western leaders say Islamic State, which has proclaimed a caliphate in the parts of Syria and Iraq it controls, now poses a greater danger.

Attacks claimed by Islamic State have prompted intensified air strikes 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 and various anti-Assad rebel groups in Syria.

Barrett said the appeal of Islamic State, also known as the Islamic State of Iraq and Syria (ISIS), could not be resolved with bombs alone.

"We have to find better ways to address the ISIS appeal," Barrett said by email. "This is not about murder and mayhem and more war, it is about the way we see each other."

He also referred to the riposte by a London bystander who shouted "You ain't no Muslim, bruv" at a suspected knife attacker as he was detained by police officers at an underground train station.

"The Leytonstone rebuke 'You ain't no Muslim, bruv' does far more to undermine ISIS than dropping bombs on Raqqa," said Barrett, who between 2004 and 2013 headed the United Nations team that monitored al Qaeda and the Taliban.

'MORE THAN WAR'

The headline estimate of 27,000-31,000 foreign fighters who have traveled to Syria and Iraq compares with Barrett's estimate for the Soufan Group in June 2014 of 12,000 foreign fighters who had journeyed to Syria.

Most of the fighters flocking to Syria are Arabs from the Middle East and Africa, although militants have attracted fighters from across 86 different countries ranging from Norway to Uzbekistan.

Significant numbers came from Tunisia, Saudi Arabia and the former Soviet Union, while about 5,000 have traveled from the European Union. There have been 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.

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.

Though some countries provide numbers of those who have gone to Syria, there are few global estimates.

The figure for France may be overstated, as officials say that 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.