UK says bomb could have brought down a plane

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 said on Saturday.

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

"I can confirm the device was viable and could have exploded. The target may have been an aircraft and had it detonated the aircraft could have been brought down," May said.

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 north of London.

"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.

"At this stage there is no information to indicate another attack is imminent. The (international terrorism) threat level in this country is at severe, that means a terrorist attack in this country is highly likely," May said, adding she did not plan immediately to change the threat level.

Direct cargo and passenger flights from Yemen to Britain were suspended in January, following an attempt to bomb an aircraft destined for Detroit, but May said more precautionary measures were needed.

"I have agreed with the Transport Secretary (Philip Hammond) to take immediate action to stop the movement of all unaccompanied air freight originating in Yemen and moving into or through the UK. We are in contact with the transport sector about this," she said.

She said she had briefed Prime Minister David Cameron on developments and would speak again to US secretary for homeland security Janet Napolitano.