A woman wearing an explosive vest threw two grenades into a crowd waiting at a checkpoint at a food distribution centre in Pakistan before setting off her suicide bomb, killing at least 45.

Officials believe it is the first suicide attack by a woman in the country.

The bomber attacked in the northern city of Bajur, a conflicted region near the Afghan border where the military has clashed with the Taliban and al Qaeda.

U.S. President Barack Obama condemned the attack, calling it an "outrageous" act which is an affront to Pakistanis.

Pakistan army helicopters and artillery killed dozens of Islamic militants in the same region Saturday.

The attack also came a day after some 150 militants killed 11 soldiers in an attack in the tribal region of Mohmand, where the army has been carrying out operations.

The suicide bomber was dressed in a woman's traditional burqa, local police official Fazal-e-Rabbi Khan told The Associated Press. An examination of human remains confirmed the bomber was a woman, officials said.

"Police asked for her identity, but she ran toward the centre and lobbed hand grenades at the police," Khan said. "She exploded herself when she reached the crowd."

The Pakistan Taliban have claimed responsibility for the attack. Many of those attacked belonged to the Salarzai tribe, who were among the first to set up a militia – known as a lashkar – to fight the Taliban.

"All anti-Taliban forces -- like lashkars, army and security forces -- are on our target," Pakistan Taliban spokesman Azam Tariq said. "We will strike them whenever we have an opportunity."

Experts said it was only a matter of time before a woman was used in an attack in Pakistan.

"It is no surprise. They can use a woman, a child or whatever," Hasan Askari Rizvi, a Lahore-based security and political analyst, told The Associated Press. "Human life is not important to them, only the objective they are pursuing."

Officials said the blast injured at least 100, as well.

With files from The Associated Press