A leading expert on terrorism says over 12,000 foreigners from 74 countries have gone to fight in Syria, 60-70 percent from other Middle Eastern countries and about 20-25 percent from Western nations.

Prof. Peter Neumann, who directs the International Center for the Study of Radicalization at King's College London, said the Syrian conflict has sparked the most significant mobilization of foreign fighters since the 1980s war in Afghanistan.

He said Monday that the Afghan conflict in the 1980s produced al-Qaida and the Syrian conflict is now forging new networks that will carry out terrorist attacks.

Neumann has been consulting the U.N. Security Council ahead of its Sept. 24 summit meeting, chaired by President Barack Obama, on foreign terrorist fighters and the threat they pose.