US officials believe that the Nusra Front, al-Qaeda's affiliate in Syria, has been working with its Yemen affiliate, AQAP, to carry out operational testing of new bomb designs in Syria.
The new explosives the bombmakers are trying to design include non-metallic bombs, ABC News reported.
There are also concerns that US and Europe-bound planes could be targeted by foreign fighters with Western passports who spent time fighting for Islamist groups attempting to overthrow the regime of Syria's president Assad.
More top news
Chelsea have won the Premier League after beating Crystal Palace 1-0 at Stamford Bridge thanks to an Eden Hazard goal
Craig Charles is quitting Coronation Street after to head back into outer space with Red Dwarf, ITV has confirmed.
The Conservatives have not ruled out increasing tuition fees above the current maximum of £9,000 a year, William Hague has said.