Wearing a paramilitary uniform, a suicide bomber gained easy entry into the otherwise highly guarded office of the United Nations’ World Food Programme (WFP) on Monday, killing at least four people and wounding five others as he detonated himself in the lobby.

The bombing shattered the relative calm of the last few months in the capital.

The UN temporarily shut down all its offices in Pakistan until further notice while it assessed the security of its staff and operations.

Interior Minister Rehman Malik said the bombing was a “desperate act” by militants “whose back has been broken” in the recent operations in Swat and elsewhere.

“They are like a wounded snake,” he said, promising that “we will launch the same kind [of] action against the [militant] left-overs as we did in Swat, Bajaur and Mohamand [tribal areas]”.

Mr. Malik said the militants had only one aim: “They want to destablise Pakistan and give it a bad name”.

Among the dead were two Pakistani women and one Iraqi national. Foreigners were also among the injured.

The suicide bomber was wearing the uniform of the paramilitary Frontier Corps.

Guards deployed at the office allowed him to enter the heavily fortified and barricaded building after he asked to use a bathroom, Interior Minister Rehman Malik told journalists during a visit to the site of the attack.

The office of the World Food Programme is located in a large house on a leafy street in the capital’s F8/3 residential area. Close to 100 staff were present in the office at the time of the explosion around 12.15 p.m.

The area is well-guarded as it houses a number of Pakistani VIPs and diplomats. The WFP office is only a couple of minutes’ walk from the private home of President Asif Ali Zardari. But he does not live there anymore, staying instead at the highly secured Aiwan-e-Sadr. The district courts are also a stone’s throw from the site of Monday’s attack.

In addition to being guarded by 19 security guards of a private security firm, two policemen and three Frontier Corps personnel, the WFP office was a well-secured fortress.

As with many diplomatic and international missions located outside the high-security Diplomatic Enclave, the frontage of the WFP building was protected by a high wall constructed of the increasingly used HESCO barriers, which are basically bags of cement encased in hardy wire mesh.

The barriers are meant to absorb the impact of an explosion directed at the building from outside, but in this case, the suicide bomber had no problem gaining entry into the building.

The bomber’s head and legs were recovered from the lobby of the building, where the police said he detonated himself.

“Once he was inside the gate, it was not difficult for him to reach the reception,” said Mr. Rehman Malik.

The minister said investigators were also studying CCTV footage to determine the chain of events leading up to the bombing,

The sound of the bomb going off could be heard around F-8/3 and windows shook, but on the street where the attack took place, there was no visible damage to other houses. All the damage seemed to be contained inside the building.

Smoke was rising over the compound for several minutes after the blast. Fire engines and ambulances fought for space in the narrow street with a media posse. Entry into the building was strictly barred for media.

"There was light and smoke and glass everywhere,” said Dominque Franke Fort, deputy-director of the WFP, who was on the first floor of the building. When he came down, he saw that “some people were not moving, there was lot of damage”.

Since the Marriott bombing in September 2009, all UN organisations based in this country have taken extraordinary security precautions, including barricading their offices with HESCO barriers, and asking families of non-Pakistani staff to leave the country.

It is not clear how badly the bombing will affect UN operations in this country, but it is unlikely that the international organisation will shut shop and leave. Insiders said more likely was the possibility that all but the most essential staff would be asked to leave.

The attack is also likely to focus attention on the demand of residents of posh Islamabad sectors that diplomatic missions and international organisations that operate out of rented houses in residential sectors are becoming a security hazard for the others who live around, and must relocate to safer areas.

Mr. Malik said the government had already asked the UN to shift all its offices to the Diplomatic Enclave.

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