At least 43 people have been killed in a suicide bombing in the town of Khar in the volatile Bajaur region in northwestern Pakistan.
More than 60 others were wounded in the attack, some seriously.
The suicide bomber attacked an aid distribution center where a large crowd of people were waiting for food rations.
U.S. President Barack Obama condemned the attack as "outrageous."
"Killing innocent people outside a World Food Program distribution point is an affront to the people of Pakistan and all of humanity," he said in a statement.
According to local police officials, the bomber first lobbed two hand grenades into the crowd outside the distribution center, before detonating an explosives-laden vest.
Witnesses say the attacker was dressed in a traditional women's burqa. The police say that an examination of human remains confirms the suicide bomber was a woman. Women rarely carry out suicide attacks in Pakistan.
"First there were two small explosions and people started running for cover. But within seconds there was a major blast and there were dead bodies scattered everywhere," one witness, Hussain Ahmed, told Reuters. "It was very terrifying."
Khan Muhammad, an eyewitness, told RFE/RL's Radio Mashaal that dozens of injured people were taken to hospital by helicopter and that the majority of victims were local people.
Another eyewitness, who did not want to give his name, described the scene of the deadly attack to Radio Mashaal: "When I went to the blast site, I saw dead bodies everywhere and it was impossible to identify them. There were body parts everywhere. We can’t explain the situation in words. I couldn't hold my tears when I saw all this."
Dosti Rehman, a hospital official, told Reuters that he has "counted 40 bodies but the death toll could rise as several wounded people are in critical condition."
Hundreds of people displaced by earlier fighting in the area had been receiving food aid at the center used by the World Food Program and other aid agencies.
The Bajaur region is located in tribal areas close to the Afghan border, known as a Taliban and Al-Qaeda stronghold. Pakistani government forces have carried out numerous military operations there to clear the area from militants. Hundreds of militants have been killed in such operations.
Victims of the December 25 attack were from the Salarzai tribe, a major regional anti-Taliban tribe, which has been backing government forces' operations against the insurgency in the area.
This latest suicide bombing comes a day after a deadly clash between militants and government forces in neighboring Mohmand region, which killed 24 insurgents and 11 soldiers.
Another suicide attack earlier this month left at least 40 people dead in Mohmand. More than 100 were killed in the same area in a double suicide attack in July.
A senior military official in October said it would take at least six months for the army to dislodge insurgents from Bajaur and Mohmand.
reported by Radio Mashaal correspondent Farkhanda Wazir; written by Farangis Najibullah, with agency reports