A female suicide bomber has killed at least 43 people in an attack on a large crowd receiving aid in north-west Pakistan, officials say.
The blast took place in the town of Khar in the Bajaur region, in tribal areas close to the Afghan border - a Taliban and al-Qaeda stronghold.
People displaced by fighting had been getting food at a distribution centre.
Reports say at least another 100 people were injured and there are fears the death toll could rise.
The attack came as Pakistan's military took action against militants in Mohmand, an adjacent tribal region, killing an estimated 40 rebels.
Saturday's bombing in Bajaur was the latest in a string of recent attacks in Pakistan's north-west.
Pakistan's Taliban said they were responsible for the attack on the distribution centre, which is used by the World Food Programme and other aid agencies.
Those in the crowd were displaced members of the Salarzai tribe, which has supported the army's operations against the Taliban.
Claiming responsibility, a Taliban spokesman said the rebels had targeted the local people because of their support for the Pakistani military.
An estimated 300 people were queuing for food at the time of the blast.
"I myself have counted 40 bodies but the death toll could rise as several wounded people are in critical condition," Dosti Rehman, an official at the main government hospital in the region, told Reuters news agency.
Dozens of injured people were taken to hospital by military helicopter.
The bomber was a dressed in a full-length burka and reportedly threw a grenade after resisting a search. A number of police officials said the attacker was actually a woman, rather than someone wearing the burka as a disguise.
From hospital, one of those injured described the moment the bomb went off.
"I was waiting to be searched in a queue at the checkpoint outside the ration point and heard a grenade explosion. People started running in panic and then a huge blast occurred," tribesman Mushtaq Khan told the AFP news agency.
Pakistani Prime Minister Syed Yusuf Raza Gillani condemned the attack and said those responsible had no regard for humanity or religion.
He said the fight against militants would continue.
The tribal district of Bajaur, where the attack took place, has seen several military operations to clear it of insurgents. The army had previously declared the operations a success and the area safe for the displaced to return to.
At least 11 Pakistani soldiers and 24 militants were killed on Friday after some 150 Taliban fighters attacked five Frontier Corps checkpoints in the neighbouring Mohmand tribal region.
Earlier this month, a suicide bomb attack in Mohmand left at least 40 people dead.
A double suicide bombing in Mohmand in July killed more than 100.
Pakistan has faced growing US pressure to launch a major ground offensive in the nearby tribal region of North Waziristan, considered a haven for Islamist insurgents.
Islamabad has denied accusations that it is not doing enough to fight the Taliban in the north-west, pointing out that more than 2,400 of its soldiers have died fighting militants since 2002.
Pakistan supported the Taliban regime in Afghanistan from 1996-2001, but later became an ally of the US when it led an invasion in 2001.