United States ground forces have begun a major attack against Iraqi Republican Guard divisions,
in what is being seen as the start of a key offensive against Baghdad.
US forces are closing on Baghdad
Around Karbala, 80 kilometres (50 miles) south of the capital, Saddam Hussein's elite force came under relentless bombardment from US forces, with correspondents reporting wave after wave of air attacks followed by repeated firing from rocket launchers.
On the other side of the Euphrates River, a second US column is progressing, while to the east, US marines say they have seized a key bridge across the River Tigris at the city of Kut, allowing them to take control of one of the main highways north towards Baghdad.
The capital itself has continued to come under bombardment into the morning, with at least six explosions reported before daybreak.
The BBC's Paul Wood in Baghdad says there is no sign yet of the advance, but that residents are fully aware that troops are trying to encircle the city.
Analysts say these operations in and around the capital are an attempt to prepare the ground for the real battle for Baghdad by drastically weakening the Republican Guard.
In Washington, officials said that the US air and artillery assault over the past few days had greatly reduced the strength of these elite troops, but there has been no independent verification of these claims.
Correspondents say that in fact the Republican Guard has yet to be fully tested, and the calibre of its weapons - as well as the tenacity of its men - is still unclear to the coalition.
In other developments
- US officials accuse Iraqi forces of firing on them from inside a revered mosque, but Iraq's information minister said American troops had attacked the site
- US special forces rescue American private Jessica Lynch, who was taken prisoner by the Iraqis on 23 March when her convoy was ambushed in the southern town of Nasiriya
- US Secretary of State Colin Powell is in Turkey to ask for more help with American military operations in northern Iraq
- The human rights organisation, Amnesty International, urges US-led forces in Iraq to take greater care to prevent civilian casualties, and called on Iraq to stop soldiers posing as civilians.
- Red Cross workers visit towns south of Baghdad for the first time and describe overwhelmed hospitals as being scenes of "horror"
- A British soldier is killed in an accident involving a light armoured vehicle - becoming the 27th UK serviceman to die in the Iraq conflict.
Communications facilities and a presidential palace were among the reported targets in Baghdad, and television pictures showed thick smoke rising over the city.
In recent days the capital has been subjected to a battering, with coalition forces taking aim at both military buildings and Republican Guard positions.
Pentagon officials, anxious not to be drawn into urban warfare in Baghdad, have said these units must be eliminated before ground troops go into the capital.
Following last night's seemingly successful attack on the Republican Guard in Karbala, the 3rd Infantry Division is now pushing towards the city, reports the BBC's Gavin Hewitt, who is travelling with the unit.
The aim of the operation was not to take the city, but merely to make it secure enough to move troops further north, our correspondent says.
Increasing numbers of US troops are now arriving in the Gulf to provide back-up to those units pushing ahead.
Some 5,000 troops from the US 4th Infantry Division are now on the ground in Kuwait but it could still be weeks before they enter Iraq, according to the division's
assistant commander, Brigadier General Stephen Speakes.
These soldiers were originally to enter Iraq through Turkey, but the plan was scuppered after Turkey refused to open its territory to US ground troops.
There are roughly 100,000 US and British troops now inside Iraq, part of a coalition force numbering about 300,000 through the region.
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Critics have suggested this is an insufficient number with which to topple Saddam Hussein, accusing military planners of trying to wage war too cheaply.
But General Richard Myers, the chairman of the US Joint Chiefs of Staff has lashed out at reports that Defence Secretary Donald Rumsfeld had pushed US commanders to go into battle with fewer troops than they wanted.
Calling the claims "bogus", he said: "This subject is not useful. It's not good for our troops, and it's not accurate."
He said the US air campaign had reduced at least two of their divisions to 50% fighting strength - but correspondents have pointed out that bombing of the Yugoslav army in Kosovo in 1999 did not weaken it significantly.
General Myers also expressed "regret" for the killing of at least seven Iraqi women and children at a checkpoint near the southern city of Najaf on Monday.
US commanders are investigating the Najaf killing.