London (AFP) - US-bound travellers from Europe and the Middle East faced tighter airport security Thursday due to fears that Muslim extremists are developing new explosives that could be slipped onto planes undetected.
The stepped-up security checks were ordered as the US embassy in Uganda warned of a "specific threat" to attack Kampala's Entebbe international airport on Thursday between 1800 and 2000 GMT.
Although the embassy did not name any group, Al-Qaeda linked Shebab insurgents have claimed recent attacks in neighbouring Kenya and Djibouti, and at home in Somalia.
US Homeland Security Secretary Jeh Johnson announced the extra security on direct flights to the United States from some overseas airports on Wednesday, without citing evidence of any specific plot.
The move comes amid broader Western intelligence concerns that hundreds of Islamist radicals travelling from Europe to fight in the Middle East could pose a security risk on their return.
On Sunday, US President Barack Obama warned that "battle-hardened" Europeans who embrace jihad in Syria and Iraq threaten the United States because their passports mean they can enter without a visa.
The airports concerned are located in the Middle East and Europe and were targeted "based on real-time intelligence", according to an official at the Department of Homeland Security who spoke on condition of anonymity.
Britain confirmed it is bolstering security at its airports in response.
-'Safety first' -
Analysts said the move was likely linked to concerns Al-Qaeda in the Arabian Peninsula (AQAP) was passing on bombmaking expertise to militants fighting in Syria.
The fear is that militants with European passports could then bring these skills back home with them and launch an attack, experts add.
"The key thing that they (the authorities) are contending with is the old adage that we have to be successful every time to prevent attacks, they need to be successful just once," Matthew Henman, manager of IHS Jane's Terrorism and Counter-Insurgency Centre, told Sky News.
Despite the increased checks, Britain said the international terror threat level issued by security service MI5 remained unchanged at substantial, the third highest grade out of five, where it has been since July 2011.
Prime Minister David Cameron said Britain was taking a "safety first" approach.
"We take these decisions looking at the evidence in front of us and working with our partners," he told Sky News.
"The safety of the travelling public must come first -- we mustn't take any risks."
The measures threaten disruptions for passengers at the start of the summer holiday season.
But officials insist passengers should not face significant delays and London's Heathrow airport -- one of the world's busiest international air hubs -- and Gatwick, south of the capital, were both operating normally Thursday.
Belgian interior minister Joelle Milquet, whose country is also stepping up airport security, told RTL-TVI that the measures would focus on electronic equipment such as tablets, computers and mobile phones.
Other aviation security experts suggested there could be an increase in random searches and the number of passengers being asked to remove their shoes at security.
"I doubt that for most people there will be that much of a difficulty, maybe queues at the airports will be a bit longer," Nigel Inkster of think-tank RUSI, a former assistant chief of MI6, told BBC radio.
Passengers in Britain have long faced tight security measures at airports following high-profile threats including a failed attempt by British "shoe bomber" Richard Reid to blow up a US-bound flight in 2001.
Security was further tightened after a plot to blow up "liquid bombs" on transatlantic flights was uncovered in 2006.
A previous high-profile attempt by AQAP to blow up a US-bound plane failed on December 25, 2009 when Nigerian Umar Farouk Abdulmutallab tried to detonate explosives hidden in his underwear.
The Detroit "underwear bomber" is now serving a life sentence in the United States.