At least 11 members of the same family - mostly children - have been killed in a coalition air strike on a residential district in central Iraq, western news reports say.
Razek al-Kazem al-Khafaji's family was in the line of fire
Hospital sources in Hilla, about 80 kilometres (50 miles) south of Baghdad, said they were among 33 civilians killed and more than 300 injured in the attack early on Tuesday morning.
US Central Command said preliminary investigation had not "turned up any evidence of this alleged incident", but officials were continuing to look into it.
On Monday, US soldiers killed seven Iraqi women and children at a checkpoint near the southern city of Najaf.
US commanders are also investigating the Najaf killing, but their first reaction was to back the troops.
Correspondents say the incidents will increase criticism of US-led troops who have already engaged Republican Guard forces defending Baghdad.
The chairman of the US Joint Chiefs of Staff, General Richard Myers, said the troops would launch their big push "as soon as we're ready".
Key military developments:
- Two Republican Guard divisions move to reinforce units defending Baghdad as they come under more intense bombardment
HAVE YOUR SAY
The strategy, if one can call it that, was flawed from the beginning
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The attack near Hilla began on Monday as part of the US-led advance towards Baghdad.
US forces said many Iraqis and at least one American were killed in fierce fighting there - the closest the clashes have been to the capital.
Hilla hospital director Murtada Abbas said the bombing had targeted the Nader residential area.
Correspondents reported seeing children wrapped in blankets on the floor of the hospital.
Among them was Razek al-Kazem al-Khafaji, who said he had lost his wife, six children, his father, his mother, and two brothers.
He said the family was fleeing fierce fighting in Nasiriya, further south, when they were attacked.
"God take our revenge on America," he was reported as saying.
Advance on Baghdad
Correspondents travelling with advance units of coalition troops say they have already clashed with units of Republican Guards some 70 kilometres south of Baghdad.
The BBC's David Willis says he has seen half a dozen Iraqi bodies by the roadside alone - the result of a fierce battle lasting 45 minutes a few miles east of the Saddam Canal.
CASUALTIES OF WAR
US: 43 military dead (including 11 in accidents, 2 under investigation), 17 missing
UK: 26 military dead (including 15 in accidents, 5 to 'friendly fire')
Iraq: Iraq: Military casualties not disclosed
Civilian casualties: Nearly 600 dead
Figures from respective governments
The ground clashes - supported with air strikes and artillery - continued as coalition aircraft kept up an intense bombardment of Republican Guard positions on the southern outskirts of Baghdad.
General Myers said US officials had seen dispersals of Iraqi troops "into neighbourhoods and things like that" - but no retreat.
"They have moved some of their units around trying to reinforce. But some of them have been degraded to pretty lower percentages of combat capability, below 50% - I think at least two [division]," General Myers said.
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denies rumours of a ceasefire negotiation, saying the only thing to discuss is Iraq's unconditional surrender
- US Secretary of State Colin Powell is in Turkey on the first foreign trip by a senior US official since the war began.
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