Pakistan blamed Taliban militants Tuesday for the deadly suicide bombing at the UN food agency's heavily fortified compound here as authorities detained some of the guards posted outside to try to determine how the attacker gained entry. The suicide bomber, who killed five people at the World Food Program's Pakistan headquarters Monday afternoon, was dressed as a security officer and was allowed to enter - apparently bypassing the normal security procedures - after asking the guards outside if he could used the bathroom. Interior Minister Rehman Malik told reporters Tuesday that the Taliban carried out the attack to avenge the Aug. 5 slaying of their leader Baitullah Mehsud in a US drone attack. "We should expect a few more (attacks)," he said. After the attack, the UN announced it was temporarily closing all its offices in Pakistan, but said its Pakistani partner organizations would continue distributing food, medicine and other humanitarian assistance. The world body said it would reassess the situation over the next several days. Malik, who was visiting those injured in the bombing at a Pakistani hospital, said the government had taken several of the guards outside the UN offices into custody for questioning as part of the investigation into the security lapse. "All the security arrangements were in place at the UN office," he said. The United Nations considers itself a major target in Pakistan. Many of its offices are surrounded by 12-foot-high blast walls. Its staff members are driven in bulletproof cars and not allowed to bring their families with them on assignment in the country. The World Food Program compound, which employs more than 70 people, is surrounded by square metal cages filled with sand and small stones used to protect against blasts and projectiles. "This was one of the best-protected UN centers in all of Pakistan," UN spokeswoman Michele Montas told reporters at the world body's headquarters in New York. "We were really quite heavily guarded at least at that compound. How that person got in - that is still being investigated, and we're trying to find out from surveillance cameras." It was unclear whether the attack Monday singled out the World Food Program because of its work in Pakistan or was simply aiming to kill foreigners or those working with them. The dead were four Pakistanis and an Iraqi. Extremists in Pakistan, Afghanistan and Iraq seeking to attack high-profile Western targets have shown no hesitation in striking foreign humanitarian agencies, including the United Nations, regardless of the work they are doing in relieving the suffering in the countries. A blast in June on a luxury hotel housing many foreign aid workers in the northwestern city of Peshawar killed two UN staffers and wounded others. The attack came a day after the new Pakistani Taliban leader met reporters close to the Afghan border, vowing more attacks in response to US missile strikes on militant targets in Pakistan. Ending speculation he had been killed, Hakimullah Mehsud denied government claims the militants were in disarray and said his fighters would repel any army offensive on their stronghold in South Waziristan. Malik said the government was already targeting Taliban militants in South Waziristan and "if needed, a further action shall also be taken at an appropriate time" against militants in other areas along the Afghan border. "We will fight all of them," he said, referring to the militant groups.