A series of blasts this week have killed more than 160 people, as U.S. and Iraqi officials warned they expected more violence before the U.S. withdrawal from cities.
American troops already have begun pulling back from the joint bases that they occupied with Iraqi security forces as part of a counterinsurgency strategy aimed at clearing volatile areas and holding them.
The recent spike in violence has raised new concern about the ability of Iraqi forces to protect the people, but a U.S. military spokesman insisted that American combat troops would be out of the cities by Tuesday as required by a security pact.
Brig. Gen. Steve Lanza said the recent high-profile attacks, which are usually blamed on Sunni insurgents, were part of an effort to rekindle sectarian violence that pushed the country to the brink of civil war.
"What's important to understand is that the people have not responded to this," he told reporters at a briefing at the U.S. Embassy in Baghdad's protected Green Zone.
In the past, bombings in Shiite areas would be followed by mortar attacks against Sunni strongholds or execution-style killings that were the signature of Shiite militias.
The explosion in Sadr City - the deadliest to strike the sprawling slum in more than two years - came just days after the U.S. military handed over to Iraqis its main base on the edge of the former Shiite militia stronghold. The strategic district was used by Shiite militants to launch rockets onto the Green Zone during the last major fighting in the city in 2008.
The bomb, which was hidden under vegetables on a motorized pushcart, exploded about 7 p.m., apparently timed to maximize casualties by striking shoppers buying food for their evening meal at the Mradi market.
Shrapnel was blown more than 600 yards away and some shops were set on fire, a police officer said.
As is usual after bombings in Iraq, there were conflicting death tolls, as victims were taken to several hospitals.
An Interior Ministry official said 69 people were killed and 135 wounded, while police and hospital officials in Sadr City put the death toll at 72.
The officials spoke on condition of anonymity because they were not authorized to release the information.
U.S. helicopters buzzed over the blast site, which was cordoned off by Iraqi police.
Qassim Ismail, 24, who was walking through the market with friends, was wounded by shrapnel.
"It sounded like unbelievable thunder and there was shattered glass and a hurricane of wind that knocked me down," he said. "I found myself lying next to a concrete block, which may have saved me."
Another bomb exploded later Wednesday in the mainly Shiite neighborhood of Jihad, killing at least one civilian and wounding 10 others, most of them young men who had been enjoying themselves at a nearby billiards hall.
U.S. and Iraqi officials anticipated the rise in violence before the June 30 deadline for most American troops to pull back from urban areas - the first stage of a full withdrawal by the end of 2011. U.S. combat operations will continue in rural areas and if requested by the Iraqi government.
Four days earlier, a truck bombing killed 82 people in a mainly Shiite town near the northern city of Kirkuk, which was the deadliest bombing so far this year.
Back-to-back suicide bombings by female attackers also killed 71 people outside a Shiite shrine in Baghdad on April 24.
Lanza said the U.S. military had recorded a declining trend in the number of high-profile attacks, with 28 in April, 16 in May and 10 this month, excluding the Sadr City blast, which occurred hours after he spoke to reporters at a briefing.
U.S. and Iraqi officials are still working out details on the number of American military personnel who will remain in the cities in an advisory and training role, Lanza said, but he stressed the main goal of ending the U.S. urban combat role was "nonnegotiable."
"We are transitioning our mission in the cities from combat operations to stability operations," he said, declining to give any information about numbers of the Americans remaining in cities except to say it would be "extremely small."
Iraqi Prime Minister Nouri al-Maliki has called the withdrawal a "great victory" and called for celebrations on Monday and Tuesday.
"I think this is the Iraqi way of asserting their sovereignty," Lanza said.