PARIS — Even if the Islamic State group is one day defeated on its territory, the world could face an even greater threat from tens of thousands of battle-hardened jihadist veterans, experts have warned.

US intelligence services estimate some 30,000 people have joined the ranks of ISIS from around 100 countries, adding to the huge number who have fought with older Islamic extremist groups over the decades.

In Afghanistan alone between 1996 and 2001, some 10,000 to 20,000 people received jihadist training, many under the guidance of Osama Bin Laden.

After the fall of the Taliban regime, many of those fighters dispersed around the world, taking their radical ideology and knowledge with them.