A small Al Qaeda [Images] cell is either on its way to the United States or may already be in the country to commit terrorist acts, senior American intelligence officials have said.

In light of the intelligence input, the White House has convened an urgent multi-agency meeting on Wednesday to consider steps needed to minimize the threat and harden the security for government buildings.

A former FBI agent and now its consultant has been quoted as saying by ABC news that they have information that a cell or cells coming to the United States want to attack government facilities.

Law enforcement officials were quoted as saying that the recent failed attacks in London [Images] have provided important new clues about possible tactics. The London attackers' use of the Internet left important clues that are being used to decode other e-mails that had initially been deemed unimportant but are now taking on new significance.

The ABC report on Tuesday came on the same day the Chicago Tribune quoted Homeland Security Secretary Michael Chertoff as saying that the nation faces a heightened chance of attack this summer.

'I believe we are entering a period this summer of increased risk,' Chertoff told Chicago Tribune's editorial board in an unusually blunt and frank assessment of America's terror threat level.

'Summer time seems to be appealing to them," he said of Al Qaeda, adding, 'We do worry that they are rebuilding their activities.'

Chertoff said there are not enough indications of an imminent plot to raise the current threat levels nationwide. And he indicated that his remarks were based on 'a gut feeling' formed by past seasonal patterns of terrorist attacks, recent Al Qaeda statements, and intelligence he did not disclose.

There is an assessment 'not of a specific threat, but of increased vulnerability,' he added.

But some experts suggest that repeatedly raising of threat level could make people complacent.

There have been reports already that suggest intelligence warnings at a similar level to the summer before September 11, 2001 and that Al Qaeda may be mobilising, the Tribune said.

'We could easily be attacked,' Chertoff said, adding, 'The intent to attack us remains as strong as it was on September 11, 2001.'

The dire warnings and Chertoff's comments come as the Bush administration faces political and business opposition over its immigration and border policies that have security implications, the paper noted.

Chertoff argued that Americans must soon decide between enduring greater inconvenience and costs or allowing terrorists easier access to the borders. He warned against increasing resistance to security measures based on comfort and self-centered motives.

'If you get to complacency then I guarantee you we will lose the race with the terrorists,' he said.