WASHINGTON – The U.S. has ordered tighter security at some foreign airports offering direct flights to the United States, the Homeland Security secretary said Tuesday.

Intelligence officials are concerned about new al-Qaida efforts to produce a bomb that would go undetected through airport security, one counterterrorism official told The Associated Press. The official spoke on condition of anonymity because the official was not authorized to discuss the matter publicly by name.

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.

Homeland Security Secretary Jeh Johnson said he directed the Transportation Security Administration to put more security measures in place at some foreign airports that have non-stop flights to the U.S. The TSA has the authority to call for additional security measures at foreign departure points for direct flights to the U.S.

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 one of the many U.S. or Western European passport holders who have travelled to Syria to fight 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.

The counterterrorism official declined to describe the bomb, but officials in the past have raised concerns about explosives being surgically implanted.

The Homeland Security Department would not say whether the call for enhanced security was in response to a specific threat.

The U.S. shared “recent and relevant” information with foreign allies, Johnson said.

“Aviation security includes a number of measures, both seen and unseen, informed by an evolving environment,” Johnson said.