The US-led occupation authorities announced on Thursday that the blast had taken place a day earlier.
No reason was given for the delay and the statement gave no further details on the circumstances or the target of the attack.
Ramadi. 100 km west of Baghdad is a volatile towm where much of the resistance to the occupation forces has been concentrated.
Elsewhere in Iraq a US military convoy came under attack near Falluja, a day after people in the town west of Baghdad killed and mutilated four US contractors, leading to the cancellation of a major trade fair.
Witnesses said a rocket propelled grenade hit a vehicle, leaving four US soldiers killed on Thursday but the casualties could not be confirmed.
On Wednesday, two vehicles carrying four American security guards working for US-based Blackwater Security Consulting were ambushed, setting the vehicles and passengers on fire.
Frustrated Iraqis dragged the corpses through the streets, beating the bodies and stringing two of them from a bridge.
US occupying administrator Paul Bremer vowed on Thursday the deaths would not go "unpunished".
The killings evoke memories of the 1993 incident in Somalia, when an American serviceman’s corpse was dragged through Mogadishu.
Residents of the Iraqi capital expressed shock, disgust and shame over the gruesome scenes.
Iraqis condemned the deaths in
Falluja a day earlier
"Mutilating a body is haram (banned under Islam). It is
unacceptable for Muslims, Christians and Jews alike. This is what the Quran (Islam's holy book) teaches us," said Khalaf, a resident of Baghdad.
Iraqi journalist Khalil al-Azzawi was outraged by what happened and dismayed that the scenes were beamed on televisions across the world.
"Television networks will exploit these ugly images in order to
portray Iraqis as criminals. But that is not the reality. Iraqis are peace loving people who wish to live in harmony with all the countries of the world," he added.
Wednesday was one of the bloodiest days for occupation forces in Iraq, with seven soldiers killed in separate attacks.
Tensions have been running high across the country since the Falluja attack.
In Baghdad Aljazeera's correspondent reported that a US soldier was seriously wounded on Thursday morning when a military convoy hit a roadside bomb on the highway, north of Baghdad.
At least one Iraqi demonstrator was shot dead by police during protests in the southern city of Basra on Thursday.
This is the second protest to erupt
in Basra this week
Clashes erupted between police and around 100 unemployed protestors, who set tires on fire and threw stones.
British occupation forces, who occupy Basra, said they had no immediate information.
And in the northern city of Kirkuk, three US occupation soldiers were wounded in rocket attacks on their base at the Kirkuk airport west of the city, said a US officer.
Kirkuk police chief Turhan Yusuf said the base was hit by five rockets "in three successive waves" that started at 9:30 pm on Wednesday and the last one at 3:00 am on Thursday (0000 GMT).
Trade fair cancelled
Amid fears that security is deteriorating, a high-profile US-sponsored trade fair for companies vying for lucrative recontruction contracts in Iraq was cancelled on Thursday.
Organisers of the Baghdad Expo, a major trade fair that had
been due to start on Monday, said it was postponed, a blow to US efforts to draw investment to Iraq and project an image of a stable country conducive to doing business.
No new date was set for the trade fair.
International companies had been due to take part in the exhibition, along with US companies that have won most of the contracts Washington has awarded to rebuild Iraq so far.
But many companies had expressed concern about security at
the event. Baghdad's main hotels have also been repeatedly attacked with mortars and rocket-propelled grenades.