"That's a deadly combination, where you have people who have the technical know-how along with the people who have this kind of fervor to give their lives in support of a cause that is directed at the United States," said US Attorney General Eric Holder in a nationally televised interview with an ABC News show on Sunday.

Holder then expressed serious concerns about the actions of such militants when they return back to their home countries.

"In some ways, it's more frightening than anything I think I've seen as attorney general," added the senior US official.

American authorities say nearly 7,000 people have travelled to Syria from the US and European countries to join the foreign-sponsored insurgency war against the Syrian government.

Holder made the remarks after US Transportation Security Administration (TSA) on July 6 introduced new enhanced security measures at overseas airports with direct flights to the United States.

US intelligence officials have warned that militants from Yemen who specialize in making bombs have traveled to Syria to link up with other insurgents there and teach them how to develop a new bomb that could go undetected through airport security.

Washington and its Middle East allies have themselves been involved in training the anti-Syria insurgent groups in a US-led regime-change plot to topple the government of President Bashar al-Assad.

Since the insurgency war began in Syria more than three years ago, many extremists from the US and other western countries have travelled to Syria.

American Intelligence officials are now worried that those insurgents holding Western passports may attempt to take the undetectable bombs into the US.

MB/MB