The revelation that militants sought to attack an airliner with an improved "underwear bomb" in a plot foiled by US and allied authorities shows their determination to build bombs that can pass through airport security, US officials said.
The Obama administration said on Monday that authorities in the Middle East recently seized an underwear bomb which they believe al Qaeda's Yemen-based affiliate had intended to give to a suicide bomber to blow up an airliner bound for the US or another Western country.
US officials told Reuters the device was seized within the last 10 days.
The plot was detected in its early stages, and no US airliner was ever at risk, officials said.
Yet the aborted plot shows that the Yemen-based group, al Qaeda in the Arabian Peninsula (AQAP), remains intent on attacking the United States or its allies, and is continuing to evolve its weapons and tactics.
One official said the latest underwear bomb to be discovered appeared to be similar to the work of fugitive Saudi militant Ibrahim Hassan al-Asiri, who US sources believe is a bomb-maker working with AQAP.
US officials said it had design features which were somewhat more sophisticated than a bomb used in two attempted attacks in 2009.
In the first incident, a man equipped with a bomb in his underwear tried to attack Prince Mohammed bin Nayef, a senior Saudi Arabian counter-terrorism official.
The bomber killed himself in the attack but the prince survived.
On Christmas Day that year, a Nigerian-born militant who had spent time in Yemen, Umar Farouk Abdulmutallab, tried to detonate a bomb sewn into his underpants as his flight from the Netherlands to Detroit began its descent in US airspace.
The device burst into flames but its explosive charge did not detonate. Abdulmutallab was subdued by fellow passengers and was later jailed by US authorities.