The US has called for tighter security measures at foreign airports amid concerns of a new al Qaida effort to create a bomb that would go undetected through security.
Officials have previously raised concerns about non-metallic explosives being surgically implanted inside a traveller's body, designed to be undetectable in body searches or to metal detectors.
The US has been planning for additional measures for the past month, a counter-terrorism official said, adding there was no immediate threat that led to the announcement by the Homeland Security Department that it was requesting tighter security abroad.
American intelligence has picked up indications that bomb makers from al Qaida's Yemen affiliate have travelled to Syria to link up with the al-Qaida affiliate there. The groups are believed to be working to perfect an explosive device that could foil airport security.
Americans and others from the West have travelled to Syria over the past year to join al-Nusra Front's fight against the Syrian government. The fear is that fighters with a US or Western passport - and therefore subject to less stringent security screening - could carry such a bomb onto an American plane.
It is not clear which airports are affected by the extra security measures, but industry data show that more than 250 foreign airports offer non-stop services to the US, including Paris' Charles de Gaulle Airport, Amsterdam's Schiphol Airport and the United Arab Emirates' Dubai International Airport.
The call for increased security is not said to be connected to Iraq or the recent violence there or upcoming July 4 celebrations.
Homeland Security Secretary Jeh Johnson said: "People should not overreact to it or over-speculate about what's going on, but there clearly are concerns centered around aviation security that we need to be vigilant about."