"We are attacking regime Republican Guard forces in the vicinity of Karbala with elements of 5th Corps, which is the 3rd Infantry Division and 82nd Airborne divisions combined," a spokesman with the US Central Command said in Qatar, requesting anonymity.

"They are attacking in concert with the 1st Marine Expeditionary Force which is attacking near Al-Kut."

The spokesman said that US forces were attacking four elite Republican Guard divisions. The Baghdad, Medina and Nebuchadnezzar divisions were being engaged by US troops south of the capital, while the Adnan division was being attacked from the air to the north.

An AFP reporter travelling with the 3rd Infantry earlier reported passing through the so-called Karbala gap to the west of the city, a strategically vital passageway for any advance on Baghdad.

"This is it, the last push," said Major Maurice Goins before the attack near the Shiite Muslim holy city, some 80 kilometres (50 miles) south of the capital.

Members of the 3rd Infantry met only "disorganised" resistance, said Colonel Will Grimsley, commander of the division's first brigade.

"It's much less worrying now we are through the gap," Grimsley said as the troops continued their push north after crossing between the city and a nearby lake.

At least a dozen prisoners of war could be seen huddled to the ground guarded by US soldiers as a convoy passed through the gap in an operation which began shortly after midnight (2100 GMT Tuesday).

The US forces fired off dozens of rounds from rocket launchers towards targets in and around Karbala as they began the push through the narrow strip of desert.

At one point fireballs lit up the sky, triggering four explosions which sent clouds spiralling some 30 feet (nine metres) into the air.

Around half a dozen bunkers could be seen on the side of the road which had been shelled.

"It's either a mortar blast or a special shell from a Bradley (tracked armoured vehicle)," said one US officer.

The sound of Bradleys firing 25mm guns could be heard in the distance, but no return fire was heard.

The troops, who have been wearing chemical warfare suits since crossing into Iraq, were ordered to don chemical boots during the drive as an indication of a higher state of alert.

Captain Andrew Valles said that engineers were also detonating mines in the area.

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

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

SPACE.WIRE