The discovery of a sophisticated and potentially undetectable bomb is reviving fears about the capabilities of Al Qaeda a week after the anniversary of the killing of its leader Osama bin Laden.

FBI experts are analysing what US officials say is a new kind of "underwear bomb". It was seized last month, probably in Yemen, when intelligence officers thwarted a plot to blow up a US-bound commercial airliner.

Details are sketchy, with officials reluctant to discuss a continuing investigation. But it is believed the bomb has no metal components and was designed to pass undetected through airport scanners.

The plot bears the hallmarks of Al Qaeda in the Arabian Peninsula (AQAP). US officials say they say the device is probably the work of Ibrahim Hassan Al Asiri.

Authorities suspect he built the original underwear bomb, which failed to detonate aboard a plane over Detroit in 2009. He is also suspected of being the bomb-maker behind the failed plot to detonate explosives hidden in toner cartridges in printers on a plane to Chicago that was intercepted in Dubai in 2010.

Some reports suggest a Saudi intelligence tip played a crucial role in unravelling the new plot.

Saudi intelligence collaboration also played a crucial role in the unravelling of the 2010 explosive toner cartridges plot.

The new bomb was designed to be used in a passenger's underwear, but this time with a more refined detonation system, US officials said.

It showed that "terrorists keep trying", Hillary Clinton, the US secretary of state, said yesterday during a visit to India. "They keep trying to devise more and more perverse and terrible ways to kill innocent people."

The revelation of the new bomp plot came after an unmanned US drone strike on Sunday killed Fahed Mohammad Ahmed Al Quso, an Al Qaeda figure charged with the 2000 bombing of the USS Cole. He was one of the FBI's 10 most wanted terrorists, and US officials now say he was a key person behind the new underwear bombing plot.

There were no immediate plans to change security procedures at America's airports, according to US authorities, but US politicians said the device may not be the only one made.

Mike Rogers, the Republican chairman of the House of Representatives intelligence committee, said what he called the premature release of information surrounding the device could complicate the investigation.

"If something bad happens because it was leaked too early, that's a catastrophe and it's also a crime," Mr Rogers said.

Dianne Feinstein, a Democrat senator and head of the Senate Intelligence Committee, described the bomb as "undetectable". It was "going to be on a US-bound airliner", she said after having been briefed by security officials.

US officials have declined to say where the Central Intelligence Agency seized the bomb. The would-be suicide bomber, reportedly based in Yemen, had apparently not yet picked a target or purchased plane tickets when the CIA seized the bomb.

FBI bomb experts, meanwhile, are "taking a look at the construction, to see what type of refinements and modifications might have been made" since the 2010 plot, John Brennan, assistant to the president on homeland security and counterterrorism, said yesterday.

Mr Brennan credited good intelligence with the unravelling of the plot, and said the device never got near an airport.