The Iraqi capital was reeling on Monday from devastating bombings which killed 41 people and exposed the limits of a month-old security sweep through Baghdad by Iraqi and US forces.
The US military also reported the deaths of five more US troops in Iraq, three of them in action and two in non-combat incidents.
Shiite families mourned the deaths of 31 pilgrims who were slaughtered in a car bomb attack as they arrived back in Baghdad after a religious festival in the holy city of Karbala.
The bodies of 12 of the slain devotees left Baghdad again in a procession of vehicles for Najaf, 160 kilometres (100 miles) south of the capital, where they will be buried.
Coffins were draped in Iraqi flags and loaded on to pick-ups and minibuses, accompanied by women in black crying and screaming in horror at their loss and fear of the trip ahead, which would take them back into danger.
"We are scared of being attacked again on the road to Najaf," said one young man.
"What wrong did they do? They just went to Karbala for the 40th day of Imam Hussein's death," said another mourner, a mother whose two sons were killed.
Millions of Shiites poured into Karbala over the weekend to mark Arbaeen, 40 days after Ashura, which commemorates the killing in 680 of Imam Hussein, a grandson of the Prophet Mohammed, by armies of the Sunni caliph Yazid.
Many were still streaming home on Monday, walking along highways exposed to attacks by insurgents, which had killed almost 200 pilgrims in bomb and gun attacks on the outward leg of the Karbala journey.
The new US commander in Iraq, General David Petraeus, has acknowledged that destroying car bomb factories is one of the more difficult tasks facing troops pouring into Baghdad as part of Operation Fardh al-Qanoon (Enforcing Law), which was launched February 14 to quell sectarian violence in the capital.
While two such factories on the outskirts of Baghdad were recently destroyed, Petraeus told reporters last week, others are proving difficult to detect because they are scattered across the rural belt surrounding Baghdad.
The US military is implementing a "surge" of more than 21,500 extra troops into Baghdad and neighbouring Anbar province in support of the plan, which has already seen the deployment of some 90,000 Iraqi and US soldiers.
US President George W. Bush at the weekend approved another 2,400 soldiers and 2,200 military police, which he said would serve as support units for the 21,500 extra troops.
Bush also called on Iran and Syria to follow up positive statements with concrete action after an international conference on Iraq, urging the two governments to stop the flow of weapons and foreign fighters into Iraq.
"If they really want to help stabilise Iraq there are things for them to do, such as cutting off weapon flows and or the flow of suicide bombers into Iraq," Bush told reporters during a visit to Colombia.
"There's all kinds of ways to measure whether they're serious about the words they utter," Bush said.
The 16-nation conference in Baghdad on Saturday endorsed a pledge to "fight terrorism and enhance security" with the United States joining its enemies Iran and Syria at the same conference table.
The conference marked the first time in months Iran and the United States had direct talks and was seen as a chance for Washington to open a dialogue with two states it has previously sought to isolate.
US commanders accuse Iranian agents of smuggling weapons to Shiite militias in Iraq, including components for lethal roadside bombs blamed for the deaths of at least 170 US soldiers since May 2004.
They also accuse Syria of allowing Sunni Arab extremists to cross its borders to join Al-Qaeda-linked groups fighting in Iraq.
The latest US military deaths brought to 3,197 the number of US troops killed in Iraq since the 2003 invasion, according to Pentagon figures.
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