The US is calling for tighter security measures at some foreign airports, as intelligence officials are concerned about a new al Qaeda effort to create a bomb that would go undetected through airport security, a counterterrorism official has said.
American intelligence has picked up indications that bomb makers from al Qaeda's Yemen affiliate have travelled to Syria to link up with the al Qaeda affiliate there.
The groups are working to perfect an explosive device that could foil airport security, the counterterrorism official said.
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.
Al Qaeda's affiliate in Yemen, called al Qaeda 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.
The counterterrorism official, who would not be named because he was not authorised to discuss the matter publicly, declined to describe the bomb.
But officials in the past have raised concerns about non-metallic explosives being surgically implanted inside a traveller'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 US counterterrorism official who was not authorised to speak publicly by name.
Another US 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 US official said.
Meanwhile, the State Department has instructed US Embassy employees in Algeria to avoid US-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 US-branded hotel," according to the message from the US Embassy in Algeria.
State Department spokeswoman Jen Psaki did not provide details about the reasons for the increased security.
"I would say broadly speaking that the threat of foreign fighters is a concern that we share with many counterparts in the world, whether that's European or others in the Western world, where we've seen an increase in foreign fighters who have travelled to Syria and other countries in the region and returning," Psaki said.
"And so we have been discussing a range of steps we can take in a coordinated fashion for some time."
The US shared "recent and relevant" information with foreign allies, Homeland Security Secretary Jeh Johnson said in a statement. "Aviation security includes a number of measures, both seen and unseen, informed by an evolving environment."
Southwest Airlines, which along with subsidiary AirTran Airways, flies between the US 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 personnel have been contacted by the Homeland Security Department. Mainz declined to comment on those discussions.
American Airlines spokesman Joshua Freed said the company has been in contact with Homeland Security about the new requirements but declined to comment further.