The defence secretary, Donald Rumsfeld, said: "What will follow will not be a repeat of any other conflict. It will be of a force and scope and scale that has been beyond what has been seen before."
The first wave of attacks, launched early yesterday morning, was the first, he said. "It will not likely be the last," he added, pledging to liberate Iraqi civilians from Saddam Hussein's regime.
He said the Pentagon was still studying damage from the opening attack. He urged Iraqi citizens to stay in their homes and told Iraqi troops to disobey any orders to use chemical weapons or destroy oil wells.
Those who surrendered would have a place in a future free Iraq but those who fought would share Saddam's fate.
As reports emerged of oilfields burning in southern Iraq, Mr Rumsfeld appealed over the head of President Saddam and instructed Iraqis not to engage in sabotage.
"Following such orders would be to commit crimes against the Iraqi people," he said. "See those orders for what they are, the last desperate gasp of a dying regime."
"The day of your liberation may soon be at hand," Mr Rumsfeld added.
General Richard Myers, the chairman of the joint chiefs of staff, played down predictions of a quick victory with few US casualties. "We do not regard combat as an easy task," Gen Myers said at the Pentagon. "Warfare is dangerous. We will have casualties."
A senior military official, speaking on condition of anonymity, said military intelligence was picking up signs and "circumstantial evidence" that President Saddam and his senior leadership were either incapacitated or out of communication with battlefield commanders. It was too early to say if they were killed or wounded, the official said.
"We are seeing no coordinated response to our first attack," the official said.
Mr Rumsfeld said initial strikes launched early yesterday had hit a senior Iraqi leadership position, but he did not confirm whether President Saddam had survived. He also claimed US intelligence services had penetrated the heart of the Iraqi military.
He said: "We are in communication with more people who are officials of the military at various levels, who are increasingly aware that it's going to happen, he's going to be gone."
Hours before the strikes a helicopter carrying US special forces crashed in southern Iraq, a senior defence official said. There were no casualties and the troops were retrieved safely.
The official also said a small plane flying from Iraq towards a marine expeditionary force position in Kuwait crashed short of its mark.
The marines were prepared for a chemical weapon attack but no agents were detected.