"It's the first time ground forces have been fully engaged against Republican Guards," said the official, who spoke on condition of anonymity.

The US push against the Republican Guard forces near Karbala, about 80 kilometers (50 miles) south of Baghdad, primarily involved the US 3rd Infantry Division, the official said, adding the battle was "ongoing."

"Since it's going against the Republican Guard, we expect it to be pretty significant," he said.

The attack followed days of intensive bombing of the Republican Guard's Medina, Baghdad, Hammurabi and Al Nida divisions, Iraq's best trained, best armed and most loyal forces.

General Richard Myers, the chairman of the Joint Chiefs of Staff, earlier said attacks from the air and ground had reduced the combat capability of two Republican Guard divisions by more than half.

Two thirds of the 800 strike sorties launched by coalition warplanes on Tuesday targeted Republican Guard divisions, and most of the remainder were in support of US and British ground forces elsewere in the country, a US defense official said, speaking on condition of anonymity.

"They are being attacked from the air. They're being pressured from the ground. And in good time, they won't be there," said US Defense Secretary Donald Rumsfeld.

Rumsfeld denied rumors of cease-fire negotiations or a third party peace plan that he said had been circulated by Iraqi officials.

"The only thing that the coalition will discuss with this regime is their unconditional surrender," he said.

The drive against Baghdad's defensive lines was launched in the face of controversy in Washington over whether the Pentagon had deployed sufficient forces to press the battle for Baghdad and protect supply lines to their rear.

Myers vehemently rejected the criticism, which has come from retired army generals and unnamed military officials, and insisted there would be no pause in the fighting.

The five Republican Guard divisions defending the capital were dispersed in defensive positions and had not retreated in the face of earlier US air and ground assaults, Myers said.

But he said they were "continuously being struck by both our ground and air forces, significantly degrading their combat capability."

US ground forces probed Republican Guard lines on Monday, triggering clashes in the Karbala area.

"I think there's bigger pushes that will be under way as soon as we're ready," Myers said.

The Adnan Republican Guard Division that was defending Tikrit, Iraqi President Saddam Hussein's hometown, has moved farther south toward Baghdad and the Nebuchadnezzar Division has moved at least two brigades from the north to reinforce divisions south of the capital.

"They have moved some of the units around trying to reinforce, but some of them have been degraded to pretty low percentages of combat capability, below 50 percent in, I think, at least two cases, and we continue to work on them," he said.

The Iraqi divisions typically have about 10,000 to 12,000 troops.

They are blocking the path to Baghdad of the 3rd Infantry Division, the 1st Marine Division and the army's 101st Airborne Division, which were maneuvering south of the city.

SPACE.WIRE