The US has ordered tighter security at some foreign airports offering direct flights to the United States, the Homeland Security secretary said.
Intelligence officials are concerned about new al-Qaeda efforts to produce a bomb that would go undetected through airport security, one counter-terrorism official told The Associated Press.
The official spoke on condition of anonymity because the official was not authorised to discuss the matter publicly by name.
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.
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 US.
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 US or Western European passport holders who have travelled to Syria to fight could carry such a bomb onto an American plane.
Al-Qaeda's affiliate in Yemen, called al-Qaeda in the Arabian Peninsula, has long been fixated on bringing down aeroplanes 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 counter-terrorism 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 US shared "recent and relevant" information with foreign allies, Johnson said.
Airport security is also being increased at British airports.
The Department for Transport says the extra measures – which have not been disclosed – are not expected to cause "significant disruption" to passengers and notes that the official UK threat status remains unchanged.
"The UK Government keeps aviation security under constant review in conjunction with international partners and the aviation industry," a DfT spokesman said.
"We have taken the decision to step up some of our aviation security measures. For obvious reasons we will not be commenting in detail on those changes.
"The safety and security of the public is our paramount concern. The UK has some of the most robust aviation security measures and we will continue to take all the steps necessary to ensure that public safety is maintained."
Prime Minister David Cameron's official spokesman said: "There are terror organisations around the world that seek to do the UK, its citizens, citizens of many other countries including our Western allies, harm.
"We need to always be vigilant to situations that can develop."
