Security was tightened around US financial institutions earlier this week after raids in Pakistan recovered documents reportedly naming them.

Homeland security adviser Frances Townsend said some of the information recovered was collected in 2000/2001.

But she added: "It appears that some of it may have been updated as recently as January of this year."

And she denied that the alert was prompted by political considerations in the run-up to November's presidential poll.

Some have suggested that the timing of the latest US government warning is designed to knock presidential challenger John Kerry off the front pages after his nomination as the Democratic Party's candidate last week.

"It had nothing to do with the Democratic National Convention," Townsend insisted.

Her comments followed reports in leading American newspapers that US officials were unsure if Osama bin Laden's network was still conducting surveillance on the sites named as potential targets.

She confirmed: "You can't tell from the intelligence itself whether or not those individuals [who amassed it] are still here."

Employees turned up for work this week despite specific US government warnings naming the New York Stock Exchange, the Citigroup Center building in New York, the International Monetary Fund and World Bank buildings in Washington DC, and Prudential Financial's headquarters in Newark, New Jersey.

The raids in Pakistan reportedly turned up hundreds of photos, sketches and written documents, which included details on the number of pedestrians passing named buildings, and whether explosives would be able to melt the steel holding them up.

A computer and communications expert reportedly linked to al-Qaeda, Mohammad Naeem Noor Khan, and one of America's most wanted terror suspects, Ahmed Khalfan Ghailani, were among those held by the security forces.

Some leading Democrats believe that the Bush administration is playing the terrorism card for all it is worth.

Whatever lies behind this heightened alert, the broader threat remains real enough and is likely to grow as November's election gets closer.

President George W Bush has described the US as a "nation in danger".

He has asked Congress to clear the way for a new national intelligence director, and announced the creation of a national counter-terrorism centre to collect and analyse data on suspected terrorist activities.

The measures follow recommendations made by the Senate commission that investigated the 11 September attacks.