Of the five victims, at least two were women, news agencies reported. One of the victims was an Iraqi who worked for the agency.
Media reports indicate that security is generally very tight at the U.N. office, which is staffed with some 80 people, and is located in an affluent residential neighborhood of Islamabad. Pakistan's Interior Minister Rehman Malik reported the bomber, a young man in his twenties, had asked a guard to use the bathroom in the office. Security tapes show him entering with a two-foot cylinder, which may have been a detonator.
"There was a huge bang, and something hit me. I fell on the floor bleeding," Adam Motiwala, an information officer at the U.N. agency, told the Associated Press.
"This is a heinous crime committed against those who have been working tirelessly to assist the poor and vulnerable on the front lines of hunger and other human suffering in Pakistan," U.N. Secretary-General Ban Ki-Moon told reporters. The U.N. has temporarily closed all of its offices in Pakistan, but Ki-Moon pledged that humanitarian aid to the country would continue.
There was no immediate claim of responsibility, but Islamist militants linked to al-Qaeda have bombed foreign and security targets in the past, including a bombing in Peshawar in June that killed to U.N. staffers, and another attack in Islamabad that killed two police officers.
Over the weekend, the new leader of the Pakistan Taliban, Hakimullah Mehsud, met with reporters, pledging that attacks would continue against the Pakistani government and the U.S. for increased drone attacks.
The meeting was also intended to quell rumors of more deaths within the Taliban leadership.
"We all are sitting before you which proves all the news about myself ... was totally baseless and false," he said.
The previous Taliban leader in the region, Baitullah Mehsud, was reported killed in a drone attack in August.
Interior Minister Malik described a weakened Pakistani Taliban as a "wounded snake," having to resort to more desperate measures like bombing the food agency, according to Reuters.
"In a matter of a few days we'll take action against them as we took in Swat, Bajaur and Mohmand," Malik said, referring to three northwestern areas where security forces have attacked and pushed back the militants. He did not elaborate.
---- Compiled from wire reports and other media sources