Story Highlights• Pentagon says 2,200 more MPs needed for Baghdad crackdown
• House "Out of Iraq Caucus" pressures Democrats for pullout plan
• At least 31 people killed in Wednesday attacks on pilgrims in Baghdad
• The strikes follow two days of attacks that left scores dead or wounded
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BAGHDAD, Iraq (CNN) -- At least 30 Iraqis were killed Wednesday when a suicide bomber detonated an explosive vest at a popular cafe in the town of Balad Ruz, a police official said.
The explosion ripped through a busy marketplace and wounded another 40 Iraqis, said Gen. William Caldwell, a U.S. military spokesman.
The marketplace was crowded mostly with Shiites, Caldwell said.
Caldwell blamed al Qaeda in Iraq, calling the cafe bombing targeting civilians "a classical AQI trademark." (Watch Gen. Caldwell describe the situation in Baghdad )
Balad Ruz is in Diyala province, about 50 miles (80 kilometers) northeast of Baghdad and 25 miles (40 kilometers) east of Baquba.
The Balad Ruz blast came as insurgents continued attacks on Shiite pilgrims en route to the holy city of Karbala.
Wednesday's attacks, which marked the third day of violence targeting pilgrims, killed 31 people in Baghdad, police said.
A suicide car bomber struck an Iraqi police checkpoint as a group of pilgrims walked along a main road in southwestern Baghdad's Saydiya neighborhood.
The bomber killed 22 people -- 12 police and 10 pilgrims, the U.S. military said. Forty-one people -- 18 police and 23 pilgrims -- were wounded, a Baghdad police official said.
Earlier Wednesday, attackers detonated a roadside bomb before opening fire on pilgrims, killing at least seven and wounding 14 others in the capital's predominantly Sunni Dora district, the official said.
In central Baghdad, gunmen also opened fire on pilgrims in the Nadha district, killing two people and wounding three others, the official said.
Iraqi security forces have been deployed into Sunni neighborhoods in Baghdad to secure the roads for the Shiite pilgrims.
The attacks came after two days of bombings and small-arms attacks that left scores dead or wounded, including at least nine killed on Monday and more than 140 on Tuesday. (Watch how pilgrims have become a popular target )
In the deadliest attack, suicide bombers targeted Shiite pilgrims Tuesday in the central city of Hilla, killing 114 people and wounding 147 others, police said. Forty of the injured are in critical condition, a Hilla police official said.
Many of the pilgrims were from Iraq's southern provinces and were walking on a main road through the city's downtown.
Pilgrims are traveling through Hilla en route to Karbala for Arbayeen, a religious gathering for Shiites that falls on Saturday.
Arbayeen marks the end of a 40-day mourning period that begins on Ashura, the anniversary of Imam Hussein's death. Imam Hussein is the grandson of the Prophet Mohammed and is considered by Shiites to be the prophet's successor.
Karbala is about 25 miles (40 kilometers) east of Hilla.
The United States vowed to stop "atrocities against the Iraqi people," according to a joint statement from Zalmay Khalilzad, the U.S. ambassador to Iraq, and Gen. David Petraeus, commander of the U.S.-led multinational forces.
"Attacking innocent pilgrims who are participating in a religious observance is unconscionable," the statement said.
More U.S. troops requested
Petraeus has requested an additional 2,200 military police to handle an expected rise in the number of detainees resulting from a security crackdown in Baghdad, Defense Secretary Robert Gates said Wednesday.
Gates noted "some very preliminary positive signs" resulting from the operation.
"We're right at the very beginning," he told a Pentagon news briefing. "But I would say that based in terms of whether the Iraqis are meeting the commitments that they've made to us in the security arena, I think that our view would be so far, so good."
Gen. Peter Pace, the chairman of the Joint Chiefs of Staff, said at the same news briefing that the number of bombings has risen in recent days, while the number of sectarian murders has dropped.
"It remains to me, potentially, that the Iraqi people do want to stop killing each other, but that the al Qaeda wants to find ways to get them to start killing each other again," Pace said.
Gates said the increased numbers of people killed in bombings was no surprise.
"We expected that there would be in the short term an increase in violence as the surge began to make itself felt, as the Baghdad security plan began to be implemented," Gates said.
The defense secretary said that the additional MPs would come on top of the 21,500 combat troops President Bush's plan is sending, and in addition to the 2,400 other troops allotted to support them.
"That's a new requirement by a new commander," Gates said of Petraeus, who took command in Baghdad last month. Without giving details, he said that other troop requests were under review at the Pentagon.
House caucus pushes pullout
News of the additional MPs came as anti-war members of the U.S. House of Representatives on Wednesday urged colleagues to set a "clear timeline" for a U.S. withdrawal from Iraq, challenging Democrats to assume the political risk of ending the war.
In a letter to party colleagues, leaders of the "Out of Iraq Caucus" accused the Bush administration of playing a game of "chicken" with the war, "where whoever acts to bring a responsible end to their failed policy will be accused of having lost Iraq."
They argued that costs of the war have become "unsustainable," damaging efforts to battle terrorism and costing the Treasury more than $8 billion a month.
The caucus claims to include about 75 House members -- nearly a third of the Democratic majority.
An explosion killed three U.S. troops Wednesday as they cleared roadside bombs along a heavily traveled route northwest of Baghdad, the military said. The number of U.S. troops killed in the Iraq war stands at 3,180. Seven American civilian contractors of the military also have died in the conflict.
CNN's Jennifer Deaton, Andrea Koppel, Basim Mahdi and Mohammed Tawfeeq contributed to this report.
An Iraqi man sits in his hospital bed in Hilla, a day after being injured in suicide bombings.