Police believed it was the first time Islamic militants have sent a woman to carry out a suicide attack in Pakistan, where the U.S.-led war in Afghanistan against al-Qaida and the Taliban insurgents continues to spill over despite Islamabad's repeated claims of victory on its side of the porous border.
The bomber, dressed in the head-to-toe burqa robes that women commonly wear Pakistan and Afghanistan, was challenged by police at a check point, officials said.
She then charged toward a group of 300 people lined up outside the food aid distribution center in the town of Khar, tossing two hand grenades before blowing herself up, officials said. The crowd was made up of people who have fled conflicts elsewhere in the area.
President Barack Obama condemned the bombing as "outrageous." In a statement released in Honolulu, Hawaii, where he was spending Christmas, Obama said, "Killing innocent civilians outside a World Food Program distribution point is an affront to the people of Pakistan, and to all humanity."
The attack in Khar, the main city in the Bajur region of Pakistan's northwest, came a day after 150 militants waged pitched gun battles against five security posts in the adjourning Mohmand tribal region to the south. The fighting, which left 11 soldiers and 24 militants dead, was an unusually strong show of strength by insurgents in border country that the military has twice claimed to have cleaned of militants.
Helicopter gunships backed by artillery continued the battle on Saturday, pounding enemy hideouts and killing another 40 militants, said Amjad Ali Khan, the top government official in Mohmand.
The tribal regions are of major concern to the U.S. because they have been safe havens for militants fighting NATO and American troops across the border in Afghanistan. The U.S. has long pressured Pakistan to clear the tribal belt of the insurgents.
The Pakistani Taliban claimed responsibility for Saturday's suicide attack in Khar, through its spokesman, Azam Tariq.
The spokesman suggested the victims may have been targeted because most of them belonged to the Salarzai tribe, which was among the first to set up a militia - known as a lashkar - to fight the Taliban in 2008. Other tribes later formed similar militias to resist the militants.
"All anti-Taliban forces - like lashkars, army and security forces - are our target," he said. "We will strike them whenever we have an opportunity."
The attack killed 45 people, including six policemen, and wounded more than 100, at least 30 critically, said Tariq Khan, a government official in the Bajur region.
Police said the victims were from various parts of Bajur who gather daily at the center to collect food tokens distributed by the World Food Program and other agencies to conflicted-affected people in the region. The people were displaced by an army offensive against Taliban militants in the region in early 2009.
Islamist militants battling the state have attacked buildings handing out humanitarian aid in Pakistan before, presumably because they are symbols of the government and Western influence.
Tariq Khan and another local official, Sohail Khan, said an examination of the human remains has confirmed the bomber was a woman.
Hasan Askari Rizvi, a Lahore-based security and political analyst, said the suicide bombing appeared to be the first carried out by a woman in Pakistan.
"It is no surprise. They can use a woman, a child or whatever," Rizvi said. "Human life is not important to them, only the objective they are pursuing" of undermining state power, he added.
Male suicide bombers often don the burqa - an Islamic dress that also covers the woman's face - as a disguise. In 2007, officials initially claimed Pakistan's first female suicide bomber had killed 14 people in the northwest town of Bannu but the attacker was later identified as a man. Islamic militants in Iraq have used women suicide bombers several times, since women in their all-enveloping robes are seen as able to pass more easily through security, especially since male security officers are often hesitant to search women.
Akbar Jan, 45, who sustained leg wounds in the bombing, said from his hospital bed that people were lining up for the ration coupons when the explosion went off.
"We thought someone had fired a rocket," he told The Associated Press. He said within seconds he saw the ground strewn with the wounded.
"I realized a little later that I myself have suffered wounds," he said. "Everybody was crying. It was blood and human flesh everywhere."
Prime Minister Syed Yousuf Raza Gilani condemned the bombing and said Pakistanis are "united against them."
Bajur is on the northern tip of Pakistan's semiautonomous tribal belt, bordering Afghanistan and the so-called "settled" areas in Pakistan. It has served as a key transit point and hideout for al-Qaida and the Taliban.
The military first declared victory in Bajur following a six-month operation launched in late 2008. But the army was forced to launch a follow-up operation in late January this year and declared victory again about a month later. Still, violence has persisted.
Associated Press writer Ishtiaq Mahsud in Dera Ismail Khan, Pakistan, contributed to this report.