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

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

It was not clear whether any of Iraq's elite Republican Guard were involved in the resistance. Elements of the Guard's Medina armoured division and other units had earlier been reported lying in wait in the area but apparently failed to engage in combat.

"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.

A US defence official had said in Washington earlier that the 3rd Infantry was involved in an ongoing battle with Republican Guard forces.

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

It 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.

WAR.WIRE