The elevation of the threat level for the financial institutions was set off by the recent arrest of a top Al-Qaeda operative in Pakistan, the BBC News Online said.

Pakistan confirmed that plans for attacks on targets in the US and UK had been found on the computer of Ahmad Khalfan Ghailani, a Tanzanian arrested on 25 July in Pakistan and wanted in connection with the 1998 US embassy bombings in East Africa.

Pakistani Information Minister Sheikh Ahmad Rashid said the discovery had been shared with the US authorities but would not confirm that it had prompted the latest terror warning.

Another factor that gripped with fear the US security departments is the arrest of a Pakistani computer engineer, who may have been involved in Al-Qaeda communication efforts, The New York Times reported Monday.

A senior American intelligence official said analysts were reviewing recently discovered documents that amount to "a potential treasure trove."

Officials emphasized that the threat information went beyond intelligence "chatter" picked up from intercepted communications or Internet traffic, which has formed the basis for past warnings.

More and more, the alerts also follow the arrest of a Pakistani-origin woman in Texas last week after she illegally crossed the Mexican border with a false passport, 7,000 dollars in cash and a plane ticket for New York, media reports said.

Four pages of a South African passport in her possession had been torn out.

A federal law enforcement official told the Times the woman, Farida Goolam Mohamed Ahmad, 48, might have been a courier sent to deliver a message or documents to someone in the United States.

"The concern was that she may be part of a team" planning attacks, a New York law enforcement official told the Times.

There was no indication of when an attack might occur, although federal officials said it would probably be in the "near term.''

Intelligence officials said they believed people associated with Al Qaeda had studied these institutions repeatedly both before and since the attacks of Sept. 11, 2001.

They collected detailed information on things like building security measures, architecture, pedestrian traffic, access ways and nearby shops that provided cover.

Officials involved with the investigation in New Jersey said suspects were found with blueprints of the Prudential site and may have conducted a "test run" for an attack in recent days, the American daily added.

While the administration has issued terrorist warnings from time to time, officials said Sunday's announcement was more dire than in the past because the threat information was highly unusual in its specificity and, in the words of one senior intelligence official, "chilling in its scope.''

After past terror warnings, critics have at times accused the Bush administration of exaggerating the threat for political purposes. But on Sunday, prominent Democrats were making that charge, and many Democrats appeared to take the threat seriously.

The Democratic presidential campaign called the increased alert level "a very serious development" that underscores the need to move aggressively to implement recommendations made by a national commission that investigated the September 11, 2001, terrorist attacks on New York and Washington.

"Nearly three years after 9/11, the breadth and extent of the commission's recommendations make clear that there is significantly more that can be done to keep America safe," Susan Rice, senior advisor to Democratic presidential nominee John Kerry for national security affairs, told AFP.

"No matter what threats we may face, the terrorists will not divide us," Rice said.