Plane bomb 'a viable device'

A bomb found on a US-bound cargo plane in Britain was "viable" and could have brought down an aircraft if it had exploded, British interior minister Theresa May has confirmed.

In response, Britain will act immediately to stop the movement of all unaccompanied air freight from Yemen into or through Britain.

In Yemen, police have arrested a woman suspected of posting the packages. Officials say she was detained in the capital, Sanaa, after being traced through a telephone number she had left with a cargo company.

Two US-bound packages from Yemen containing explosive material were intercepted in Britain and Dubai on Friday, in what US President Barack Obama called a "credible terrorist threat".

The parcels were bound for two places of Jewish worship in Chicago the president said. The device found in Britain was taken off a cargo plane at East Midlands airport.

"We do not believe that the perpetrators of the attack would have known the location of the device when it was planned to explode," May said, speaking after chairing a meeting of the British government's "Cobra" crisis committee.

Britain would continue to work closely with international colleagues to find out more about the plot and bring to justice those responsible, she said.

May also confirmed she had briefed Prime Minister David Cameron on developments and would speak again to US Secretary for Homeland Security Janet Napolitano.