An AFP correspondent traveling with the Marines said they were backed by artillery and two B-52 heavy bombers in their drive to take a key canal near the farming town of Hilla, 80 kilometers (50 miles) south of Baghdad.

Officers said they took some 50 Iraqis prisoner after encountering stiff resistance in fighting on day 13 of their campaign to oust Iraqi President Saddam Hussein.

But a local hospital director said 33 civilians, including children, were killed and 310 wounded in air strikes earlier Tuesday against a residential area at the southern outskirts of the farming town.

Fifteen members of a family were also killed late Monday when their pick-up was blown up by a rocket from an Apache helicopter near Hilla, the sole survivor of the attack told AFP Tuesday.

The Americans took another public relations hit Tuesday when troops fearful of a fresh suicide attack opened fire on a civilian vehicle at a military checkpoint, killing seven women and children.

The early morning incident at Najaf, 60 kilometers (36 miles) south of Hilla, occurred three days after a suicide bombing killed four US soldiers and left other troops jittery.

Officials said the victims Tuesday were in a vehicle that failed to stop despite warning shots fired by US troops. Four people in the minibus escaped unharmed.

On the marines' advance, Major Cal Worth said the Iraqis put up a fight but could not hold them off. "There was good resistance initially," he said. "But again we finished them up. We continue to push on according to schedule."

Scores of tanks and armored personnel carriers were seen rumbling towards the canal near Hilla.

An intelligence officer put the size of the Iraqi force at between 300 and 400, a mixture of regular armed forces and militia loyal to President Saddam Hussein.

The marines are part of a general push towards the capital to topple Saddam.

Further west, the US Army's 3rd Infantry Division and 101st Airborne Division are also gearing for a move against elite Republican Guard units defending the approaches to Baghdad.

US officers said Iraq had brought up reinforcements for the Republican Guard units defending the approaches to Baghdad.

US commanders have said their troops were as close as 100 kilometersmiles) of the capital. Some have signaled a major tank battle could be shaping up in the next week.

Officers said mixed and matched elements of five Republican Guard divisions, elite troops backed by heavy armor, manned a southern arc in front of Baghdad, bracing for a decisive drive in the nearly two-week-old campaign.

There was no immediate estimate how many troops this would represent but it could run into the tens of thousands. Iraqi divisions normally comprise 12,000-15,000 troops.

Major John Altman, intelligence officer for the 3rd Infantry Division's First Brigade, said the Iraqis were reinforcing the Republican Guard after days of punishing air strikes and probing ground attacks by US-led forces.

"What's happening is that you reinforce the unit you expect to be in contact and you fill a unit that is attrited," Altman told AFP near Najaf, about 150 kilometers (95 miles) south of Baghdad.

He said the Republican Guard's armored Medina Division had been heavily battered by days of air strikes, artillery attacks and assaults by the tank-busting Apaches.

The Iraqis brought down from north of Baghdad elements of the armored Hammurabi and mechanized Nebuchadnazzar divisions, Altman said. Troops of the Al-Nida armored and Adnan divisions were also in place south of the capital.

"Elements of five Republican Guard divisions are oriented along the southeast and southwest of Baghdad," Altman said.

The intelligence officer reported heavy fighting around a key bridge that crosses the Euphrates River southwest of the Shiite Muslim city of Karbala, 80 kilometers (50 miles) south southwest of Baghdad.

"We made great headway," Altman said. "We can easily own the ground there and use the bridge."

Further south, the commander of the 101st Airborne Divsion's Aviation Brigade, Colonel Greg Gass, told AFP US forces were "very close" to securing the important city of Najaf, but added it could still take a few more days.

US-British forces kept up their bombardment of Baghdad Tuesday, hitting the main presidential palace complex for the second consecutive day.

Iraqi officials said 18 people had been killed and more than 100 wounded in British and US air strikes on the capital since Monday evening.

The southern outskirts of Baghdad were pounded by an especially intense bombardment that sent balls of fire and towers of black smoke into the sky, an AFP journalist in the Iraqi capital said.

SPACE.WIRE