A homicide bomber struck a crowd rushing to help schoolgirls trapped in a bus by an earlier bombing Monday, killing at least 31 people — the deadliest in a string of blasts that raise doubts about Iraqi security forces as the U.S. prepares to reduce troops.

The Interior Ministry, which provided casualty figures, said another 71 people were wounded in the twin blasts, the deadliest attack in Baghdad in six weeks. A third bomb exploded several hundred yards from the scene in the mostly Shiite Kasrah section of north Baghdad but caused no casualties, police said.

No group claimed responsibility for the attacks during the morning rush hour. But suspicion fell on Al Qaeda in Iraq, which has made homicide bombings against Shiite civilians its signature attack.

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In recent weeks, there has been an uptick in small-scale bombings in Baghdad. An Associated Press tally showed at least 19 bombings in the city this month as of Sunday, compared with 28 for all of October and 22 in September.

At least 44 people were killed in Baghdad bombings between Nov. 1 and Sunday, compared with 95 for October and 96 in September, the AP count showed.

Most of the bombings occurred during the morning rush hour — targeting Iraqi police and army patrols, government officials heading for work or commuters, in an attempt to undermine public confidence.

The neighborhood where Monday's bombings occurred is predominantly Shiite, but it is part of the largely Sunni district of Azamiyah, which had been an Al Qaeda stronghold until Sunni tribes broke with the terror movement last year.

Col. John Hort, who commands U.S. troops in Azamiyah, called the attack a "despicable, cowardly act of terrorism against peaceful people" with "absolutely no military connotation."

An Interior Ministry official speculated that extremists may have sought to "send a message" to President-elect Barack Obama about "the real situation in Iraq," pressure the government not to sign a new security agreement with the United States or embarrass the ruling parties ahead of regional elections in January.

The official spoke on condition of anonymity because he was offering speculation.

Police said the first bomb went off as a minibus passed by carrying young girls to school along a busy commercial street lined with shops and small restaurants popular with local people for breakfast.

Associated Press Television News video showed the vehicle riddled with shrapnel and the interior smeared with blood. Girls' shoes were scattered about the floor.

Witnesses said the homicide bomber mingled among rescuers and stunned bystanders, then detonated an explosive belt which probably accounted for most of the casualties. Police officials giving the toll were unclear how many died in each blast.

Baghdad hospitals and the Interior Ministry, which controls the police, provided the same casualty figures of 31 dead and 71 wounded. But the Iraqi military's Baghdad city command said only four people were killed and 35 wounded, figures disputed by witnesses and police.

The blasts shattered storefronts along the street and set more than a dozen cars on fire.

"I rushed to the site and saw several girl students trapped in a bus and screaming for help," said Abbas Fadhil, 45, who was working in a nearby restaurant.

"We took the girls outside the bus and rushed them to the hospitals," he said, standing in front of the damaged restaurant — his white shirt soaked with blood.

"This is a criminal act that targeted innocent people who were heading to work and school while the politicians are busy with their personal greed and ambitions," Fadhil said.

Ahmed Riyadh, 54, owner of a nearby grocery, said the bombing was a "vicious attack" that "did not differentiate between Shiites and Sunnis."

"We are fed up with such attacks and we want only to live in peace," he said. "The politicians should work hard and set aside their differences to stop the bloodshed."

Also Monday, a 13-year-old female homicide bomber reportedly attacked a security checkpoint in downtown Baqouba, 35 miles northeast of Baghdad, killing five people including a local leader of Sunni group opposed to Al Qaeda, police said. Fifteen other people were wounded in that explosion.

Violence is down significantly in Baghdad since the worst of the Sunni-Shiite fighting in 2006 and 2007 and the U.S. troop surge of last year. But attacks in Baghdad continue daily, albeit at a lower level than in previous years.

A string of explosions Sept. 28 in mostly Shiite areas of Baghdad killed at least 32 people and wounded nearly 100. At least 63 people were killed when a car bomb blew up in Hurriyah, a mainly Shiite neighborhood in Baghdad.

U.S. commanders have repeatedly warned that the improved security is fragile because threat groups have been weakened but not defeated. Rival religious and ethnic groups have still not reconciled their political differences that have fueled the nearly six-year conflict.

That has raised concern about the ability of Iraqi security forces to maintain security on their own after the Americans leave. U.S. troops are to leave Iraqi cities by June 30 and withdraw entirely from the country by 2012 under a still unratified security agreement.

Obama promised during the campaign to remove all American combat troops within 16 months of his inauguration Jan. 20. However, he pledged to consult with the Iraqi government and U.S. commanders before taking decisions on a drawdown.

The continuing attacks show the determination of extremist groups to continue the fight against the U.S.-backed government and lie behind U.S. military concern about drawing down the 151,000-member U.S. military force too quickly.