Addressing a special news conference at the FBI headquarters, both Ashcroft and Mueller called for heightened public vigilance, but said there was no immediate need to step up the five-colour-coded nationwide terrorist alert, which remains at the third level, yellow.

There was "credible intelligence from multiple sources" indicating a new imminent attack by al-Qaeda, said Ashcroft. "This disturbing news shows a particular intention to hit the United States hard. Beyond this intelligence, al-Qaeda's own public statements suggest that it is almost ready to attack the United States," he said.

Some of the major upcoming events —  like next month's G-8 summit in Georgia, and the Democratic national convention in July at Boston and the Republican convention in New York in August —  are potential targets, they said.

Ashcroft also warned that the al-Qaeda has modified its tactics to counter the new security measures: "The ideal al-Qaeda operatives may now be in their late 20s or early 30s and may travel with a family to lower their profile. Our intelligence confirms al-Qaeda is seeking recruits who can portray themselves as Europeans."

Pointing to pictures of seven suspects, including a woman, who could be part of an attack plot, Mueller said, "unfortunately, we currently do not know what form that threat may take. And that is why it is so important that we locate seven individuals."