The US has ordered tighter security at some foreign airports offering direct flights to the US, the homeland security secretary has said.
Jeh Johnson said he had directed the Transportation Security Administration to put more security measures in place at some foreign airports that have non-stop flights to the US.
He said the measures would try to avoid "unnecessary disruptions to the traveling public".
The US shared "recent and relevant" information with foreign allies, Johnson said, but his department would not say whether the call for enhanced security was in response to a specific threat.
The airports affected were located in the Middle East and Europe, an unnamed homeland security official told the AFP news agency.
US 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, according to the AP news agency.
US officials fear that one of the many US or western European passport holders who have travelled to Syria to fight could carry such a bomb onto a US plane.
Americans and others have travelled to Syria over the past year to join the Nusra Front's fight against the Syrian government.
Al-Qaeda in the Arabian Peninsula has previously been blamed for planning to use bombers with explosives designed to hide inside underwear and explosives hidden inside printer cartridges shipped on cargo planes.