The US Secretary for Homeland Security, Tom Ridge, has defended his decision to step up security threat levels in response to information found in coded emails on the computer of an al-Qaeda suspect in Pakistan.
Security has been tightened around financial institutions in three cities following the discovery of the information.
However, it has emerged that much of the information about the threat of another terrorist strike on US and British targets was gathered before the 11 September attacks.
Mr Ridge said the intelligence was a reminder that the US was still fighting the war on terrorism.
Earlier, the US President George W Bush described the United States as a 'nation in danger'.
Meanwhile, the British Home Office has said that it is monitoring what it calls the real and serious threat of a terrorist attack in the UK, but insists there are no details of specific targets.
It is understood that American banks and financial institutions in London, such as Merrill Lynch, are being advised to be extra vigilant.
Arrests of al-Qaeda suspects
In a separate development, the authorities in Pakistan say a suspected al-Qaeda member with a multi-million dollar reward on his head was arrested in the past few days.
Pakistan's Interior Minister Faisal Saleh Hayat declined to give any further details.
He said that two other al-Qaeda suspects caught in the last 24 hours were of African origin.
Earlier, it was reported that an intensive swoop on al-Qaeda cells in the east of Pakistan led to the arrests of least 18 al-Qaeda suspects, including four African operatives and a computer mastermind.
