Stopping Laden 'greatest challenge': Bush

Bush confirmed US reports that bin Laden had asked his chief ally in Iraq, Abu Musab al-Zarqawi, to plan attacks in the United States -- a warning that al Qaeda was still a threat to Americans at home.

"Recently, we learned that Osama bin Laden has urged the terrorist Zarqawi to form a group to conduct attacks outside Iraq, including here in the United States," Bush said at a public swearing-in ceremony for Michael Chertoff, the new head of the Department of Homeland Security.

"Bin Laden's message is a telling reminder that al Qaeda still hopes to attack us on our own soil," Bush said. "Stopping him is the greatest challenge of our day."

Shortly after the Sept. 11, 2001, attacks, Bush famously declared he wanted bin Laden caught "dead or alive," but in the years since he has been more sparing in bringing up the al Qaeda leader's name in public.

Bin Laden has eluded US efforts to hunt him down, and he occasionally surfaces in a video or audiotape to show followers he is still alive. U.S. intelligence agencies believe he is hiding in the border region between Afghanistan and Pakistan.

"We're on a constant hunt for bin Laden. We're keeping the pressure on him, keeping him in hiding," Bush said.

Zarqawi, blamed for orchestrating insurgent attacks against US forces and Iraqis, has become the most hunted man in Iraq. In December, an audiotape message purportedly from bin Laden formally named Zarqawi as the head of al Qaeda in Iraq.