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.
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Mladic, now 75, was the military commander of Bosnian Serb forces in the 1990s, who fought against the Bosnian Croat and Bosniak armies.
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His representatives said Cassidy, who was hospitalised last week, died "surrounded by those he loved."