The attack appeared to be the first suicide bombing staged by a woman in Pakistan, and it underscored the resilience of militant groups in the country's tribal belt despite ongoing military operations against them.
The bomb hit the main city in Bajur, a region near the Afghan border where the military has twice declared victory over Taliban and al-Qaida insurgents. It also came a day after some 150 militants killed 11 soldiers in a coordinated assault in a neighboring region where the army also has carried out operations.
The bomber, dressed in a traditional women's burqa, first lobbed two hand grenades into the crowd waiting at a checkpoint outside the food aid distribution center in the town of Khar, local police official Fazal-e-Rabbi Khan said. The attacker then detonated her explosives vest, he said.
Khan said the victims were from various parts of the Bajur tribal region 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.
Local government official Tariq Khan said the blast also wounded 60 people wounded, some critically.
Although women rarely become suicide bombers in Pakistan, Tariq Khan and another local official, Sohail Khan, said an examination of the human remains has confirmed the bomber was a woman.
Akbar Jan, 45, who sustained leg wound in the bombing, said from his hospital bed that people were lining up for the ration coupons when something exploded with a big bang.
"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.
Bajur and other parts of 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 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 in the region.
The army also has taken steps to clear Mohmand, a tribal region next to Bajur that also has witnessed militant activity. On Friday, however, around 150 insurgents attacked five security checkpoints in that region, killing at least 11 soldiers and wounding a dozen more in a show of their ongoing strength.