American forces fired an artillery barrage at targets in southern Baghdad on Sunday while Iraqi rescuers scoured wreckage for the victims of another deadly car bomb that left more than 70 dead.
Tehran has meanwhile confirmed that it will send its foreign minister to an international conference next week in Egypt to discuss the turmoil in Iraq, after several weeks of diplomatic pressure from Baghdad.
As the sun rose over the war-torn capital, a series of massive detonations could be heard from southwestern districts, where Iraqi officials said a US operation was under way in support of the city's joint security plan.
"Eighteen rounds of artillery were fired from Forward Operating Base Falcon," said US spokesman Lieutenant Colonel Chris Garver, without identifying the target of a salvo that could be heard 10 kilometres (six miles) away.
The death toll from Saturday's suicide bombing near a revered shrine in the Shiite pilgrimage city of Karbala rose to 71 overnight as scores of victims suffered in hospital wards crowded with 178 wounded.
City health spokesman Salim Khadhim told AFP that the dead included five women, five children and eight victims burned so entirely that their age or gender could not be determined.
"There was a two-year-old Iranian girl among those killed, and four Indian men. Two Iranian men were wounded," he added. Karbala is a site of pilgrimage for Shiites from around the world.
US command in Baghdad reported that fighting and roadside bombs had claimed the lives of seven more American soldiers and two marines over two days of intense violence, bringing the month's toll to 91.
With two days left, April is already the seventh deadliest month for coalition forces since a US-led invasion force overthrew dictator Saddam Hussein in March 2003, only to come under attack from a violent insurgency.
The scale of the carnage, more than 10 weeks after the start of a major US and Iraqi security plan, will serve as another blow to the authority of Iraq's Prime Minister Nuri al-Maliki and US President George W. Bush.
Bush has seen domestic support for his war strategy collapse, and this week he is expected to veto a bill passed by Congress to tie further military funding to a plan to begin withdrawing US troops in October.
The rage in Karbala after Saturday's attack -- in which protesters were quicker to blame the failure of the Iraqi security forces than the bomber -- underlined Maliki's fragile authority.
The blast followed a pattern suggesting an attempt by Sunni extremists to foment sectarian violence. The suicide bomber detonated his payload near a gate to the Ibn Abbas mosque, a revered Shiite shrine.
But while the Shiite-led government blames all of Iraq's many problems on "Saddamists and takfiris" or Sunni extremists, the enraged crowd that thronged Karbala pointed to corruption and incompetence.
"Most people in Karbala expected such an attack by terrorists after we heard that they are fleeing Ramadi and Baghdad. Karbala is near to these cities," said restaurant owner Muhammed Hameed.
"I do not know what measures the police took after a previous explosion a fortnight ago in a popular market," he said, referring to a near identical attack that killed 42 people.
Cafe owner Ali Mohammed al-Wazny echoed his astonishment, demanding: "Where are our intelligence and security services? Can't they find out how this vehicle entered the city and went past the checkpoints?
"This implies the security forces are infiltrated and must be replaced."
The US military said it detained 72 suspects linked to the Al-Qaeda network in a series of raids on Sunday in the provinces of Anbar and Salaheddin, the biggest daily tally of detentions in many weeks.
Iran's President Mahmoud Ahmadinejad called Maliki confirming Tehran would take part in the international conference on security this week in Egypt, Maliki's office said.
Iran had delayed confirming it participation, expressing anger over the presence of the United States at the meeting.
Washington accuses Tehran of fomenting trouble in Iraq by supplying weapons and training to extremist factions. Tehran denies these charges.
