Australia's UN Ambassador Gary Quinlan told the UN Security Council that countries in North Africa, the Middle East and Europe are already grappling with the reality of returning "battle hardened foreign fighters."
He said there is a "trend towards ever-increasing recruitment of foreign fighters" by al-Qaida and its affiliates in "a number of theaters of operation," particularly in Syria.
A recent report by UN experts monitoring sanctions against al-Qaida pointed to large numbers of foreigners who have joined Jabhat al-Nusra, an affiliate of al-Qaida in Iraq that has been fighting against Syrian President Bashar Assad's government.
His report to the council follows last week's Security Council decision officially declaring Boko Haram a terrorist group linked to al-Qaida.
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It imposed sanctions against the Islamic extremists who have carried out a wave of deadly attacks and the recent abduction of nearly 300 schoolgirls in Nigeria.
President Barack Obama told West Point graduates today that as the threat of terrorism has shifted from a centralised al-Qaida to an array of affiliates, the US must adapt.
As an example of al-Qaida's adaptability, Quinlan pointed to the movement of militant members of al-Qaida in the Islamic Maghreb away from Mali and Algeria to southern Libya where they have regrouped.
The recent report by UN experts also highlighted the generational shift in al-Qaida's leaders, with top posts being taken up by men in their late 30s and 40s who come to the job with new philosophical perspectives and techniques to reach out to potential followers, he said.