There is no indication that such a bomb has been created or that there's a specific threat to the U.S., but the Obama administration on Wednesday called for tighter security measures at foreign airports that have direct flights to the U.S.
American intelligence has picked up indications that bomb makers from Yemen-based al-Qaida in the Arabian Peninsula have traveled to Syria to link up with the al-Qaida affiliate there, known as the Nusra Front, according to a counterterrorism official who spoke on condition of anonymity because he was not authorized to speak publicly about the matter by name. The enhanced security measures have been in the works for the past month, he said.
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 be hidden inside underwear and explosives secreted inside printer cartridges shipped on cargo planes.
Over the past year, Americans and others from the West have traveled to Syria to join the fight against the Syrian government. The fear is that fighters with a U.S. or other Western passport, who therefore are subject to less stringent security screening, could carry such a bomb onto an American plane.
The counterterrorism official declined to describe the bomb. 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 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.
"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," Homeland Security Secretary Jeh Johnson said late Wednesday on MSNBC.
British airports stepped up security after the reports.
Authorities said in a statement that passengers "should not experience significant disruption" and that there would be no change in the threat level. "The safety and security of the public is our paramount concern. The UK has some of the most robust aviation security measures and we will continue to take all the steps necessary to ensure that public safety is maintained."
Deputy Prime Minister Nick Clegg told ITV that Britain was working with the United States and other countries, "so that where credible new threats are identified a response is then implemented in airports around the world."
"The hope is that the majority of travelers will not be unduly disrupted, but I hope also that people will understand that we have to work together across the world to deal with people who want to inflict harm," Clegg said.
Meanwhile, the State Department has instructed U.S. Embassy employees in Algeria to avoid U.S.-owned-or-operated hotels through July 4 and the Algerian Independence Day on July 5.
"As of June 2014 an unspecified terrorist group may have been considering attacks in Algiers, possibly in the vicinity of a U.S.-branded hotel," according to the message from the U.S. Embassy in Algeria.
The U.S. shared "recent and relevant" information with foreign allies, Johnson said in a statement Wednesday. "Aviation security includes a number of measures, both seen and unseen, informed by an evolving environment."
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.
Southwest Airlines, which, along with subsidiary AirTran Airways, flies between the U.S. and Mexico and the Caribbean, doesn't expect the directive to have much impact on its operations, spokesman Chris Mainz said. He said the focus likely would be in other parts of the world, although the airline's security people have been contacted by the Homeland Security Department. Mainz declined to comment on those discussions.
American Airlines spokesman Joshua Freed said the airline has been in contact with Homeland Security about the new requirements but declined to comment further.
Associated Press writers Eric Tucker and Joan Lowy in Washington, Danica Kirka in London and David Koenig in Dallas contributed to this report.