Washington (AFP) - US Attorney General Eric Holder said on Sunday that intelligence suggesting Islamist bombmakers from Yemen have teamed up in Syria is "more frightening than anything" he had seen before.
In an interview with ABC News, Holder called the alleged cooperation between expert bomb-makers in Yemen and jihadists fighting in Syria's civil war a "deadly combination."
ABC News, citing unidentified sources, said US intelligence suspects Yemeni bombmakers in Syria have designed an explosive device small enough to fit in a laptop computer.
"I think we are at a dangerous time," Holder said, referring to the link-up of experts with technical know-how and "people who have this kind of fervor to give their lives."
"It's something that gives us really extreme, extreme concern," he added in the interview broadcast on Sunday. "In some ways, it's more frightening than anything I think I've seen as attorney general."
Holder's comments come in the wake of increased security for flights heading to the United States.
Earlier this month, US authorities announced new security measures for air passengers from Europe and the Middle East. One such measure was that smartphones and electronic devices must be able to be switched on before travel.
The tighter security was introduced due to fears that militants linked to Al-Qaeda are developing new explosives that could be slipped onto planes undetected.
ABC cited a source at the US Department of Homeland Security as as saying the unspecified threat was "different and more disturbing than past aviation plots."
In November 2010, the Yemen-based Al-Qaeda in the Arabian Peninsula (AQAP), claimed responsibility for a plot to send parcel bombs to the United States.
The group also claimed it put a bomb aboard a UPS cargo plane that crashed two months earlier in the Gulf emirate of Dubai, killing the craft's two pilots.
The man thought to be behind that plot, master Al-Qaeda bombmaker Ibrahim al-Asiri, is currently believed to be hiding out in Yemen's restive southern provinces.
Al-Asiri, a 32-year-old Saudi citizen, specializes in building hard-to-detect non-metallic explosives, often using Pentaerythritol tetranitrate, or PETN, and chemical detonators.
US President Barack Obama warned last month that "battle-hardened" Europeans who embrace jihad in Syria threaten the United States because their passports mean they can enter the country without a visa.