Along with some 22,000 foreign fighters in Syria and Iraq, there were also 6,500 in Afghanistan and hundreds more in Yemen, Libya, Pakistan and Somalia.

At a meeting of the 15-member Security Council chaired by President Barack Obama in September, the experts were asked to report within six months on the threat from foreign fighters joining ISIS, which has declared a caliphate in Syria and Iraq, Nusra Front in Syria and other al Qaeda-linked groups.

“For the thousands of (foreign fighters) who traveled to the Syrian Arab Republic and Iraq … they live and work in a veritable ‘international finishing school’ for extremists as it was in the case in Afghanistan during the 1990s,” the experts wrote in their report submitted to the council late this month. “Those who eat together and bond together can bomb together.”