ISLAMABAD (Reuters) - A woman covered in a head-to-foot burqa carried out a suicide bombing that killed more than 40 people in Pakistan, government officials said on Sunday, adding to security challenges confronting the U.S. ally.

Any increased use of women as bombers may complicate efforts by Pakistani security forces to stem a spreading wave of Islamist suicide attacks because it is harder to spot and search burqa-clad attackers in conservative tribal society.

Saturday’s bombing illustrated the resilient ability of militants to stage attacks despite army offensives against them.

The woman blew herself amid a crowd of men, women and children heading toward a food distribution center of the World Food Program in the Bajaur region on the Afghan border.

“Initially there was confusion as to whether the attacker was a man or woman but now we have established that (it) was a woman,” senior government official Sohail Ahmed told Reuters.

Government officials in Bajaur said they had recovered the head, burqa and clothes of the bomber.

PREVIOUS WOMAN BOMBER IN 2007

It was the second such attack by a female militant in Pakistan. In the first episode, a woman detonated explosives near a military checkpost in the northwestern city of Peshawar in 2007, but she killed no one except herself.

On Saturday, the woman initially threw hand grenades at people heading toward the food center to receive aid before blowing herself up. Forty-three people were killed and more than 60 were wounded in the attack.

“If militants use more women for such attacks then it is going to be a very huge problem for the security forces,” said Rahimullah Yusufzai, an expert on tribal and militant affairs.

“They don’t have enough women (in the) police force and even (if) they have policewomen, because of our conservative culture, people don’t want their women to be subjected to body searches. It’s going to be a big problem.”

The attack happened a day after battles between security forces and insurgents in the neighboring Mohmand region that killed 11 soldiers and 40 insurgents, the government said. Militants disputed the official death toll.

The Pakistani army has conducted a series of offensives in its lawless Pashtun tribal belt, known as the global hub of Islamist militants, in recent years, killing hundreds of militants and destroying many of their bastions.

But the insurgents have still been able to strike back and have kept up a campaign of suicide and bomb attacks across the country, killing hundreds of people.

“The militants’ strongholds have been smashed...they are on the run and that’s why they are now hitting soft targets,” Prime Minister Yusuf Raza Gilani told reporters in comments broadcast by local television.

Saturday’s attack targeted members of Salarzai, a major pro-government tribe backing army offensives against militants. Salarzai tribesmen have been a key role in mobilizing lashkars, or tribal militia, to back government military operations.

A Taliban spokesman, Azam Tariq, claimed responsibility for the suicide bombing, saying that it was retaliation for “Salarzai activities against the Taliban.”