WASHINGTON >> Intelligence officials are concerned about a new al-Qaida effort to create a bomb that would go undetected through airport security, a counterterrorism official said Wednesday, prompting the U.S. to call for tighter security measures at some foreign airports.

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 this 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.

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.

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.

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 traveled 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 U.S. 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 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 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.

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Associated Press writers Eric Tucker, Lara Jakes and Joan Lowy in Washington and David Koenig in Dallas contributed to this report.