"There has been communication between bin Laden and Zarqawi, with bin Laden suggesting to Zarqawi the US homeland as a target," said the official, who spoke on condition of anonymity.

The official called the bin Laden communication "a fairly recent development" but declined to provide details for fear of compromising US anti-terrorism efforts.

The Department of Homeland Security said it issued a classified intelligence bulletin over the weekend warning state officials that the federal government had received nonspecific information about al-Qaeda plans to attack the United States.

Department spokesman Brian Roehrkasse said the threat was still being analysed, but was not enough to raise the US terrorism alert level, which is currently set at yellow to signify an elevated threat.