A "mass casualty attack" on the United States, which would rival the September 11 terror strikes, is being planned by the Al-Qaeda network, US intelligence officials were quoted as saying.

"The attack will be large-scale," an intelligence official told The Washington Times on Tuesday.

Intelligence reports state that any major attack is likely to be preceded by smaller-scale strikes, including assassinations of prominent people in the United States.

"We continue to be in a heightened threat environment," said a official who had access to the reports.

Though preparations for such an attack have been disrupted after arrests of terrorists during the past months, recent intelligence reports indicate that communications among clandestine cells of Al-Qaeda members are being restored gradually, the officials said.