Five United Nations World Food Program workers have died following a suicide bomb attack in Islamabad on Monday. Several other people have been injured, some critically.

The blast happened at 12:15 p.m. local time, shattering windows and leaving victims lying in pools of blood, witnesses said.

Security footage showed the bomber walking into the lobby holding a long cylindrical object in one hand. A few seconds later the deadly blast started with a bright flash.

Adam Motiwala, an information officer at the food agency, was injured in Monday's blast.

"There was a huge bang, and something hit me. I fell on the floor bleeding," Motiwala said, after being hospitalized with head, leg and rib injuries.

The suicide bomber was dressed as a security officer and detonated his eight kilograms of explosives in the lobby of the WFP headquarters in the Pakistani capital. He gained access to the lobby by asking if he could use a washroom located inside the building, said Pakistani Interior Minister Rehman Malik.

Hospital officials said five people died as a result of the blast, and the WFP confirmed that five of its employees had been killed, including an Iraqi national and four Pakistani citizens.

A finance assistant, an IT officer, a senior finance assistant, a receptionist and an office assistant were killed, the WFP said.

WFP Executive Director Josette Sheeran said the entire humanitarian community was grieving the loss of the "humanitarian heroes" who worked for her agency.

UN Secretary-General Ban Ki-moon condemned Monday's attack, calling it "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.S. Ambassador Anne Patterson equated the attack on the Islamabad aid workers to an attack against Pakistani society.

"Such cruel acts expose the true nature of the terrorists' agenda," Patterson said.

No one immediately took claim for Monday's attack, though it occurred one day after Hakimullah Mehsud, the newly declared leader of the Pakistani Taliban, vowed to strike back at U.S. and Pakistan in retaliation for a series of recent missile strikes.

In the past year, unmanned drones have carried out more than 70 air strikes on targets in Pakistan, resulting in the deaths of militant leaders, as well as sympathizers and civilians.

Mehsud's own brother, Kalimullah, died in an attack last month.

Pakistani leaders publicly condemn the attacks, though it is widely believed that the government supports them and likely provides intelligence so they can be carried out.

American officials have said they are considering using additional drone attacks, as well as the use of special forces, to take on al Qaeda and Taliban targets on the Pakistan side of the Afghanistan-Pakistan border.

Pakistan has pushed back its Taliban insurgency in the northwestern Swat Valley in recent months and intelligence officials say a similar campaign is planned against al Qaeda and Taliban militants in the South Waziristan region.

With files from The Associated Press