The counterterrorism official, who would not be named because he was not authorized to discuss the matter publicly, declined to describe the kind of information that triggered the warning. But officials in the past have raised concerns about non-metallic explosives being surgically implanted inside a traveler's body, designed to be undetectable in pat-downs or metal detectors.
The U.S. has been planning for additional measures for the past month, a counterterrorism 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 traveled to Syria to link up with the al-Qaida affiliate there. The groups are working to perfect an explosive device that could foil airport security, according to the counterterrorism official.
Americans and others from the West have traveled to Syria over the past year to join al Nusra Front's fight against the Syrian government. The fear is that fighters with a U.S. or Western passport _ and therefore subject to less stringent security screening _ could carry such a bomb onto an American plane.
Al-Qaida's affiliate in Yemen, called al-Qaida in the Arabian Peninsula, long has been fixated on bringing down airplanes with hidden explosives. It was behind failed and thwarted plots involving suicide bombers with explosives designed to hide inside underwear and explosives hidden inside printer cartridges shipped on cargo planes.
It wasn't clear which airports were affected by the extra security measures, but industry data show that more than 250 foreign airports offer nonstop service to the U.S., including Paris' Charles de Gaulle Airport, Amsterdam's Schiphol Airport and the United Arab Emirates' Dubai International Airport.
The call for increased security was not connected to Iraq or the recent violence there, said a second U.S. counterterrorism official who was not authorized to speak publicly by name. Another U.S. official, also speaking on condition of anonymity, said the increased security measures had nothing to do with the upcoming July Fourth holiday or any specific threat.
The extra security is out of an ``abundance of caution,'' the U.S. official said.
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