Forces are 50 miles from Baghdad

American forces have moved within 50 miles of Baghdad today as they advance on the city. Apache helicopter gunships have taken on a formation of T72 tanks, the Iraqis' most advanced armour.

From 50 miles south of the city, near the town of Karbala, the gunship helicopters fired Hellfire anti-tank missiles and their heavy machine guns at what are thought to be Republican Guard tanks.

The engagement, which lasted for three hours, is the closest yet to Baghdad, and shows how far allied forces have advanced through Iraqi territory.

One television reporter who witnessed the firefight said the Apaches had come under "heavy, heavy barrage" and it was impossible to tell whether all had returned safely.

CNN's Karl Penhaul, who was aboard a Black Hawk helicopter, riding about 15 miles from the scene of the strike said the pilots of his helicopter reported that the Republican Guard had laid up a heavy barrage in the attack which began at midnight and lasted three hours.

Lieutenant General John Abizaid of US Central Command said a fake surrender near Nassiriya, a crossing point over the Euphrates north-west of Basra, set off the "sharpest engagement of the war thus far". He added: "We, of course, will be much more cautious in the way that we view the battlefield as a result of some of these incidents."

The march to Baghdad also left as many as nine Marines dead in the fighting near Nassiriya. A dozen US soldiers were believed have been taken prisoner.

Apaches have a range of less than 200 miles from their base, suggesting they may now be operating out of Iraqi territory as US-led forces come close to the Iraqi capital. Infantry and armoured units are known to be 100 miles south of Baghdad as they advance.

Their columns, some of them miles long, include tankers to ensure the M-1 Abrams tanks and Bradley fighting vehicles which are leading the charge can keep moving.

Close air support is being provided by the Apache gunships, as well as Cobra attack helicopters and strike aircraft which are flying ahead of the advance through the desert.

Although US leaders declared the invasion on target despite the bloody setbacks on the third day of the campaign, any expectation that Iraqi defenders would simply fold was gone.

"Clearly they are not a beaten force," said General Richard Myers, chairman of the Joint Chiefs of Staff. "This is going to get a lot harder."

Elsewhere, central Baghdad shook under the weight of fierce bombing by US-led forces. Several huge explosions rocked the city centre before dawn with no prior warning from air raid sirens and no sign of anti-aircraft fire.

The blasts were among the biggest to hit the city since US and British bombardments began on Thursday.