A suicide bomber blew himself up in the lobby of the United Nations World Food Program office in the Pakistani capital, Islamabad, killing at least five people and wounding several others, officials said.

Police and security forces survey the World Food Program building in Islamabad after a suicide bomb attack on Monday. (Reuters)

The suicide bomber, who was believed to be in his 20s, was disguised as a paramilitary officer and had asked permission from a security guard to enter the building to use a restroom Monday, Interior Minister Rehman Malik said.

He detonated about eight kilograms of explosives when he reached the lobby, Malik said.

Security camera footage broadcast on local TV shows the bomber walking through a door into what appears to be the main building carrying a long cylindrical object before a bright flash fills the screen.

The blast shattered windows in the heavily fortified compound, which is situated in an upper-scale residential neighbourhood where President Asif Ali Zardari keeps a home. Witnesses reported seeing victims lying on the ground in pools of blood.

"There was a huge bang, and something hit me. I fell on the floor bleeding," said Adam Motiwala, an information officer at the UN agency who was hospitalized with injuries to his head, leg and ribs.

Officials at local hospitals said at least five people brought in from the blast site have died.

Two of those killed were Pakistani women and another was an Iraqi national, officials said.

Several others World Food Program employees were injured, two of them critically, the UN organization said in a news release.

The incident has caused some officials to raise questions about the security of the building. Visitors to UN buildings in Islamabad are usually screened and patted down for weapons and explosives in secure chambers away from the building.

The attack came a day after the new leader of the Taliban in Pakistan, Hakimullah Mehsud, vowed fresh assaults against Pakistan and the United States in retaliation for the increased number of drone-airplane attacks in the Tribal Areas on the border with Afghanistan. But there was not an immediate claim of responsibility for the bombing.

Al-Qaeda and Taliban militants have carried out scores of suicide attacks in Pakistan over the last 2½ years, a number of them targeting foreigners and their interests. Under U.S. pressure, Pakistani security forces are targeting the extremists in their strongholds in the northwest.

UN closes offices

UN Secretary General Ban Ki-moon condemned the bombing.

Pakistani anti-terrorist squad officers stand outside the offices of the World Food Program in Islamabad on Monday. (B.K.Bangash/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," Ban said in Geneva.

The UN has temporarily closed all its offices in Pakistan because of the attack.

Though the offices will be closed, the delivery of food aid to refugees will not be halted, said WFP spokesman Amjad Jamal.

The World Food Program is distributing food to poor Pakistanis, including those in the northwest who have been displaced or affected by a recent Pakistani army offensive against militants in the Swat Valley.

"This is a terrible tragedy for WFP, and for the whole humanitarian community in Pakistan," said the agency's deputy executive director, Amir Abdulla, speaking from its headquarters in Rome.

With files from The Associated Press