Al Qaeda terrorists may be plotting more urgently to attack the United States to maintain their credibility and ability to recruit followers, the U.S. military commander in charge of domestic defense said Thursday.

Air Force Gen. Gene Renuart, chief of the U.S. Northern Command, also told reporters he has not seen any direct threats tied to the U.S. presidential elections. But he said it would be rash to think that such threats are not there.

"We need only to look at Spain and see that they're certainly willing to try to do something that is significant that could affect an election process," Renuart said. "I think it would be imprudent of us to let down our guard believing that if there's no credible threat that you know of today, there won't be something tomorrow."

While he said that U.S. authorities have thwarted attacks on a number of occasions, he said terrorist cells may be working harder than ever to plot high-impact events. He did not point to any specific intelligence that authorities have received but said the "chatter" they are hearing "gives me no reason to believe they're going to slow down" in their efforts to target the U.S.

"If an organization like that is to maintain credibility and continue to grow more of its extremists, it has to show tangible results," Renuart said. "So I think there may be a certain sense of urgency among that organization to have an effect. So it would tell me that they're trying harder."

Of the more than a dozen daily events that Northern Command responds to -- ranging from natural disasters to threats -- two or three may have the potential to be terrorist incidents, he said.

The chatter, which included public audio and video tapes released on the Internet by Al Qaeda leaders, suggests that they are looking for a way to have a big impact again, he said. Pressed for details, he said the chatter was more common but "whether that's louder or more ominous, I'm not sure I'm ready to draw that conclusion."

He did, however, repeat his assertion -- which he first made last July -- that he believes there are Al Qaeda cells or sympathizers within the United States.

President Bush, in a speech, also said the United States remained under threat from terrorists. Marking the fifth anniversary of the creation of the Homeland Security Department, Bush said that in the aftermath of the Sept. 11 attacks "it was hard to imagine that we would reach this milestone without another attack on our homeland."

Yet he said, "On this anniversary, we must also remember that the danger to our country has not passed. Since the attacks of 9/11, the terrorists have tried to strike our homeland again and again. We've disrupted numerous planned attacks -- including a plot to fly an airplane into the tallest building on the West Coast and another to blow up passenger jets headed for America across the Atlantic Ocean."

Bush said the lesson is clear: "The enemy remains active, deadly in its intent -- and in the face of this danger, the United States must never let down its guard."

Asked about the terror threat, Homeland Security spokesman Russ Knocke said, "There continues to be no credible information telling us about an imminent threat to homeland at this time."

Last July, U.S. intelligence analysts, in a threat assessment, concluded that Al Qaeda has rebuilt its operating capability to a level not seen since just before the Sept. 11 terrorist attacks. The report said the terror network has regrouped along the Afghan-Pakistan border, but it also noted that officials knew of no specific credible threat of an attack on U.S. soil.

About the same time, Homeland Security Secretary Michael Chertoff raised eyebrows when he said he had a "gut feeling" that the United States faced a heightened risk of attack.