The U.S. is increasing security measures at airports overseas amid deepening concern that militant outfits in the Syria-Iraq region and Yemen may have developed next-generation bombs that could evade conventional screening and be smuggled onto commercial planes.

In a statement on Wednesday Homeland Security Secretary Jeh Johnson did not give information about a specific threat, but only said that the change in policy came even as the U.S. “continually assesses the global threat environment and re-evaluates the measures we take to promote aviation security.”

While the Secretary said that the Department of Homeland Security would implement enhanced security measures in the coming days at certain overseas airports with direct flights to the U.S., he promised that they would ensure that these steps posed “as few disruptions to travellers as possible.”

A number of measures would be introduced, officials said, and they would be “both seen and unseen, informed by an evolving environment.”

Media reports said sources told ABC News that the U.S. was planning to issue a series of “urgent directives to airport authorities and airlines operating in Europe and elsewhere, requiring them to further scrutinise U.S.-bound passengers’ electronics and shoes, set up more explosives detection machines, increase random screenings of travellers, and take a series of secret actions the public would never see.”

In the past U.S. officials have drawn attention to concerns surrounding non-metallic explosives that could be surgically implanted inside a passenger’s body, designed to be undetectable in both pat-downs and metal detectors.

This week’s measures may in part be a response to escalating tension in Syria and Iraq, where groups such as the Islamic State of Iraq and Syria, Syrian al Nusra and al-Qaeda have recently reasserted their dominance in various parts of the territory.

Similarly in Yemen Al-Qaeda in the Arabian Peninsula is said to have “been fixated on bringing down airplanes with hidden explosives… inside underwear and… printer cartridges shipped on cargo planes.”

Fears that these groups may plan strikes against targets on U.S. soil have been exacerbated by intelligence reports earlier this year suggesting, “more than 100 Americans have joined the jihad in Syria to fight alongside Sunni terrorists there.”

Addressing the Senate Foreign Relations Committee earlier this year Matthew Olsen, Director of the U.S. National Counter-Terrorism Centre, said there were thousands of foreign fighters in Syria and hundreds of them held Western passports.

Mr. Olsen was also quoted saying that a group of “al-Qaeda veterans from Afghanistan and Pakistan have gone to Syria, making the prospect of recruiting new members for the organisation even more likely.”

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