President Bush made rare mention of Usama bin Laden (search) on Thursday, calling efforts to block the terrorist leader's hope of attacking America again "the greatest challenge of our day."

Bush insisted the so-far unsuccessful hunt for the Al Qaeda (search) founder is "keeping the pressure on."

Bin Laden's trail has gone cold more than three years after the Sept. 11, 2001 attacks, and Bush hardly ever utters the name of the man he once declared wanted "dead or alive" and repeatedly promised would be caught.

But bin Laden made the headlines again this week, as intelligence officials said that he has enlisted the help of Abu Musab al-Zarqawi (search), the top Al Qaeda figure in Iraq, to plan new attacks inside the United States.

At the ceremonial swearing-in for the new secretary of the Department of Homeland Security, Mike Chertoff (search), Bush confirmed that contact between bin Laden and al-Zarqawi.

"We're on a constant hunt for bin Laden. We're keeping the pressure on him, keeping him in hiding. And today Zarqawi understands the coalition and Iraqi troops are on a constant hunt for him as well," Bush said at the Ronald Reagan federal building near the White House.

"Bin Laden's message is a telling reminder that Al Qaeda still hopes to attack us on our own soil," he said. "Stopping him is the greatest challenge of our day, and under Mike's leadership we will do everything in our power to meet that challenge."

Bush proclaimed progress in other areas of protecting the nation from attack, saying that the global campaign against terrorist cells (search) is succeeding and that "extraordinary measures" have been taken to beef up domestic protection. Both points have been hotly disputed by others.

The president acknowledged there is much more to do.

"We cannot afford to become complacent," Bush said. "As we adapt our defenses, the terrorists will adapt their tactics in response. ... They continue to pose a great threat to the American people."

Earlier Thursday, Bush spoke by phone with Iraq's interim prime minister, Ayad Allawi (search).

White House spokesman Scott McClellan said they talked about the political process in moving from the Iraqi elections to the formation of a new government "and the importance of making sure that is an Iraqi process."

They also discussed "the need for Syria and Iran to stop interfering in internal Iraqi affairs," McClellan said.

Chertoff was first sworn in at the White House on Feb. 15 by Harriet Miers, the counsel to the president, hours after the Senate confirmed him by a 98-0 vote.

Chertoff, 51, has promised to balance protecting the country with preserving civil liberties as head of the sprawling agency that was created as a result of the Sept. 11, 2001, terror attacks.

"Mike is wise and he is tough — in a good way," Bush said.