Osama bin Laden ordered al Qaeda's leader in Iraq, Abu Musab al-Zarqawi, to form a cell in 2005 to plot attacks outside the country and make the United States his main target, according to a senior US official.
Citing newly declassified intelligence, Fran Townsend, President George W Bush's adviser for homeland security, said the information backs the administration's claim that US troops must stay in Iraq for now to prevent it from becoming a "terrorist sanctuary".
Mindful of its trouble selling its war strategy to the American public, the White House is trying to put the spotlight on bin Laden's connections to Zarqawi, the head of Iraq's al Qaeda wing who was killed in a US air strike in June 2006.
Mr Bush's critics accuse him of trying to play down the role of sectarian fighting in Iraq's chaos and justify an unpopular war by focusing on links to bin Laden and al Qaeda.
The administration has abandoned earlier charges that al Qaeda had ties to the government of Iraqi leader Saddam Hussein before he was toppled in a 2003 US-led invasion.
Ms Townsend spoke to reporters on the eve of a Bush speech at the US Coast Guard Academy to give an update on the war on terrorism, and as congressional Democrats temporarily backed off on demands Iraq war funding be tied to a troop pullout timetable.
She said US intelligence officials had pieced together accounts of some of Zarqawi's dealings with bin Laden, who has eluded US-led efforts to track him down.
"The intelligence community tells us that in January 2005 bin Laden tasked Zarqawi ... to form a cell to conduct attacks outside Iraq and that frankly America should be his number one priority," she said.
"We know from the intelligence community that Zarqawi welcomed the tasking, claimed he already had some good proposals." She declined, however, to specify any potential targets and would not say whether the plotters had advanced beyond the discussion stage.
Al Qaeda has been behind some of the bloodiest bombings in Iraq since the invasion.
Townsend said that in the spring of 2005 bin Laden also told Hamza Rabia, then al Qaeda's top operations man, to brief Zarqawi on the group's "external operations planning, including homeland plots [targeting the United States]."
She said she could go public with the information now because Zarqawi and Rabia are dead and several other key al Qaeda operatives are in US custody at Guantanamo Bay, Cuba.
Ms Townsend said it was "safe to assume" detainees involved in the alleged plot had been interrogated but declined to say what methods were used and whether it yielded useful information.