As the sun rose over Baghdad, a series of massive detonations could be heard from southwestern districts, where Iraqi security officials said a US operation was under way in support of the capital'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 10km away.

Meanwhile, the death toll from Saturday's suicide bombing near a revered shrine in the Shia 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 at the city hospital 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 Shias 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.

"Three Task Force Marne soldiers were killed and one was wounded when their patrol was struck by a roadside bomb southeast of Baghdad today," the US command in Baghdad announced in a statement.

Another Task Force Marne soldier was killed in a similar incident on the same day. The US army unit patrols a violent belt of small towns immediately south of Baghdad.

"Three soldiers and two marines assigned to Multinational Force West were killed on April 27 while conducting combat operations in Al Anbar Province," another statement said.

Anbar province in western Iraq is a hotbed of Sunni insurgents.

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 Ibn Abbas mosque, a revered Shia shrine.

"There are two dangers threatening Iraq, namely al-Qaeda and the Saddamists," Maliki said in a statement.

But while the Shia-led government blames all of Iraq's problems on "Saddamists and takfiris," or Sunni extremists, the enraged crowd that thronged Karbala pointed to corruption and incompetence.

"Do the officials of Karbala not see what happens in the city?" asked resident Hussain Mahdi, as the crowd surged around burnt-out cars and scattered corpses, seeking loved ones or crying out in rage.

"Karbala has become unsafe because of the weakness of the security forces, because politicians interfere in it in order to support their own loyalists," he charged, reflecting a popular sentiment.

"A lot of people appointed in security services get their jobs after paying bribes. How do you expect such police to work for you?" asked Abdul Hadi Gowaid, who saw the explosion rip through Ibn Abbas street just before nightfall.

Iran's top national security official Ali Larijani was due in Baghdad for talks on this week's international conference on Iraqi security, the Iranian foreign ministry spokesman said.

Tehran has so far not confirmed its presence for the two-day conference starting on May 3 in the Egyptian Red Sea resort of Sharm el-Sheikh, although Iraqi officials have been trying to persuade Tehran to take part.

The United States accuses Iran of fomenting trouble in Iraq by supplying weapons and training to extremist factions -- a charge Tehran denies -- and Iran would prefer that US officials not attend the talks.

Washington is in the process of flooding an extra 28,000 troops into Iraq, while stepping up a programme to train and equip Maliki's embattled Iraqi security forces to help them meet the insurgent threat.

General David Petraeus, US commander in Iraq, said last week that murders by sectarian death squads, which policy-makers see as the greatest threat to Iraq's long-term stability, are down by two-thirds over two months.

But car bombings, and hence civilian casualties, have been relentless.

"So we have not seen a corresponding drop in the level of violence statistics that we have seen in the sectarian murder statistics," Petraeus told reporters, adding that "the level of violence has generally been unchanged."