News that the attack at In Amenas was apparently led by a Canadian appeared to confirm reports that the region, especially the northern areas of Mali that are now controlled by violent Islamists, has become a magnet for radicals from all over the world.

The Maghreb Emergent website had already quoted one Algerian worker at the gas installation as saying that the kidnappers included Libyan, Egyptian and Syrian radicals.

"Several of the group that are holding us speak Arabic with foreign accents," the anonymous Algerian said. "I have heard Egyptian and Tunisian accents and even one person who spoke in a Syrian accent."

Volunteers are also travelling to the region from Europe, according to experts. Around 20 radical Islamists have disappeared from Spain in recent months and are thought by the country's security services to have joined fighters in Mali, according to La Razón newspaper.

Fernando Reinares, a terrorism expert at Madrid's Elcano Royal Institute, said that the violent Islamist groups in the region had been recruiting in Europe and offering armed training for at least five years, but that numbers travelling there had increased dramatically since they grabbed control of part of Mali.

"Some have compared the situation to Afghanistan or Somalia, but I think the proper analogy is with the tribal zones of northern Pakistan," he said. "We have a territory where different groups have come together and evicted the state's authority. Once that is done, it becomes a focus for like-minded people." He added:

"There is now a high degree of co-ordination and organisation, and they have been highly effective in imposing themselves on a large population."

Authorities in France, Belgium, Spain, Italy and Germany had been aware that the groups operating in the region had been recruiting in Europe – mainly among immigrants from the region or second or third-generation native Europeans with roots there.

"Some of them have stayed in the region. Others have come back to Europe and others have gone on to other scenarios like Nigeria, Somalia, the Middle East and southern Asia," said Reinares.

There was now a growing danger that some would return and carry out attacks in Europe.