Plane travellers who can switch on their mobile phones or other electronic devices will be able to take them aboard as part of new security measures, US authorities said Sunday.
US-bound travellers from Europe and the Middle East have faced tighter airport security in recent days over fears that Al-Qaeda-linked militants are developing new explosives that could be slipped onto planes undetected.
The checks focused on electronic items such as laptops and mobile phones, amid fears that extremists such as Al-Qaeda could use them as their latest tactic in a long campaign of attacks involving aircraft.
"During the security examination, officers may also ask that owners power up some devices, including cellphones," the US Transportation Security Administration said in a statement, noting that all electronic devices are screened by security officers.
"Powerless devices will not be permitted onboard the aircraft. The traveller may also undergo additional screening."
The agency noted that it could "adjust" security measures further in order to provide maximum security to travellers.
A TSA spokeswoman declined to confirm further details about the enhanced screenings.
French and British authorities have urged passengers to allow extra time to get past the additional measures, which were not specified but were believed to focus on footwear and electronic items such as mobile phones and computers.
The Department of Homeland Security, under which the TSA falls, is also asking that airlines and airport authorities in Europe and elsewhere examine the shoes of passengers headed for the United States and increase random screenings of travellers, ABC News reported.
It cited one source as saying the unspecified threat was "different and more disturbing than past aviation plots."
"We will work to ensure these necessary steps pose as few disruptions to travellers as possible," DHS Secretary Jeh Johnson said.
On Wednesday, US officials had publicly demanded enhanced security for airports in Europe and the Middle East with direct US flights to the United States.
They did not say whether they had intelligence about a specific plot, but their actions suggested alarm.
The request was "based on real-time intelligence," according to a Homeland Security Department official who spoke on condition of anonymity.
Western intelligence services are concerned that hundreds of militants travelling from Europe to fight in the Middle East could pose a security risk on their return. Most European passport-holders do not need a visa to travel to the United States.
Of particular concern is Al-Qaeda in the Arabian Peninsula (AQAP), the Yemen-based branch of the terror network founded by the late Osama bin Laden.
US and other intelligence services say AQAP is passing on sophisticated bombmaking expertise to militants fighting in Syria for use against Western targets - most prominently, passenger aircraft.
France announced Friday it was boosting passenger screening at its airports, responding to a request from Washington.
The French move, to come into force Monday and Tuesday, follows similar action already implemented by Britain, and notably impacts Europe's two busiest hubs, Heathrow and Charles de Gaulle.
Combined, an average 2.5 million passengers a day pass through the two airports.
The increased security screening will not affect those travelling to the US from Australia.