At least 41 people were killed when a female suicide bomber detonated her explosive vest in a crowded aid distribution centre in north west Pakistan today.
Dozens more were injured while waiting for food stamps.
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.
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 neighbouring region where the army has also carried out operations.
The bomber, dressed in a traditional women's burka, first lobbed two hand grenades into the crowd waiting at a checkpoint outside the food aid distribution centre in the town of Khar. The attacker then detonated her explosives vest.
The victims were from various parts of the Bajur tribal region, where people gather daily at the centre to collect food tokens distributed by the World Food Programme and other agencies to conflict-affected people in the region. The people were displaced by an army offensive against Taliban militants in the region early last year.
Islamist militants battling against the state have attacked buildings handing out humanitarian aid in Pakistan before, presumably because they are symbols of the government and Western influence.
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.
He said: "It is no surprise. They can use a woman, a child or whatever. Human life is not important to them, only the objective they are pursuing."
Male suicide bombers often don the burka as a disguise. In 2007, officials initially claimed Pakistan's first female suicide bomber had killed 14 people in the north west town of Bannu but the attacker was later identified as a man.
Akbar Jan, 45, who sustained leg wounds in today's bombing, said from his hospital bed that people were queuing for ration coupons when something exploded.
"We thought someone had fired a rocket," he said. Within seconds he saw the ground strewn with the wounded.
"I realised 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 semi-autonomous 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.