The number of countries from which they have come has increased from 81 to 86, while Soufan also gives a figure for a "rate of return" - fighters coming back to Western Europe and elsewhere. This, it says, stands at 20-30 per cent.
Most of the jihadists who have flocked to Syria have been from the Middle East and Gulf countries (8240) or North Africa (8000). However, 5000 are thought to have come from Western Europe, around double the number 18 months ago.
The fastest growing group comes from Russia and other former Soviet bloc countries. Russian nationals - including many Chechens - number 2400, similar to Saudi Arabia (2500) but well behind Tunisia, which has sent the most at 6000-7000.
The rise in numbers suggests that the atrocities carried out by Isis in the last 18 months have only added to the group's appeal and that the loss of territory to the Kurds in the north has failed to damage morale. That suggests motives for joining up may be personal rather than political, the report said.
"The majority of [Isis'] video production appeals to those who seek a new beginning rather than revenge for past acts," the report says. "A search for belonging, purpose, adventure and friendship appear to remain the main reasons for people to join the Islamic State, just as they remain the least addressed issues in the international fight against terrorism."
The numbers travelling from Central Asia are also rising fast, showing Isis' ability to foster new recruiting grounds. Around 300 are said to have come from China. It has long been accused of driving its Uighur Muslim minority into the arms of jihadists.
A new video uploaded to the Isis media arm, Al-Hayat Media Centre, contained a chant in Mandarin, suggesting China is a recruitment target.
China has avoided intervening in the Middle East and has opposed foreign military action in Syria. But President Xi Jinping has called for closer co-operation on terrorism.
Attacks claimed by Isis have prompted intensified air strikes on the militants by a US-led coalition.
Barrett said the appeal of 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.
- AAP
