A report from the UN's Al-Qaeda sanctions committee raised specific fears of large numbers of Al-Qaeda affiliated foreign fighters teaming up with Syria's branch of Al-Qaeda, Al-Nusra Front.
"Ties are established that the monitoring team predicts could lead to new pan-Arab and pan-European networks of extremists," said the head of the committee, Gary Quinlan.
"Furthermore, the return of these battle hardened foreign fighters to their countries of origin, or to third countries, with new ideas and skills is a cause for concern," said Quinlan.
He told the Security Council that Al-Qaeda is getting younger, with men in their 30s and 40s taking up leadership positions, increasingly shaped by contemporary events rather than the 1990s.
For example, a new generation in Nigeria's Boko Haram are more disposed towards violence and less tolerant of local religious leaders, said Quinlan, also Australia's ambassador to the UN.
Mid-level commanders in Al-Qaeda affiliates in Africa and Asia bring "technological knowledge and a focus on innovative attack planning," he said.
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Younger leaders are also more adept at connecting with the next generation of recruits through "sophisticated use of social media," he added.
If Al-Qaeda is organizationally more splintered, a more diverse and localized recruitment makes it more durable, Quinlan said.
He described improvised-explosive devices as Al-Qaeda's weapon of choice and said affiliates disseminated step-by-step guides "in a deliberate attempt to arm 'lone-wolf' terrorists."
US President Barack Obama on Wednesday promised to ramp up support for rebels fighting in Syria's more than three-year civil war, in which more than 160,000 people have been killed.
The Wall Street Journal said Obama was ready to approve training missions for select rebel groups in a bid to counter the rising power of Al-Qaeda-linked extremists.
The UN Al-Qaeda sanctions committee last week blacklisted and imposed sanctions on Boko Haram, a month after it claimed the mass kidnapping of more than 200 schoolgirls.