A powerful explosion was heard in the northwest of Pakistan's capital city of Islamabad just past the noon on Monday as a suicide bomber struck the office of the United Nations World Food Program (WFP).

The road to the office of WFP has been closed by local police when Xinhua reporters rushed to the blast scene. As cars are not permitted to be near the spot Xinhua reporters managed to pass the security checkpoint on foot and reached the front of the damaged building.

Pakistani security officials and rescue workers survey the site of a bomb blast in Islamabad October 5, 2009. [Xinhua/AFP]

The smoke and smell of gunpowder is spreading over the blast site. Fire engines, ambulances and police cars are on stand-by on the narrow road by the office of WFP.

The two gates of the house have been closed with anti-terror police keeping high alert there.

Tariq Khan with WFP participated in the rescue of the injured and his arms and clothes are still stained with blood. He told Xinhua that he did not know the exact casualties of the blast but he confirmed that at least three UN employees had been killed including one Iraqi national and two Pakistani women.

Tariq said that the explosion took place near reception desk of the ground floor of the WFP building while about 20 people were working on the same floor. About 80 people were working in the WFP building at the moment of the blast. He said he did not know the nature of the blast and could not confirm whether it was a suicide bombing or the explosives were previously planted there.

The workers of the WFP have been evacuated after the blast and police have started investigating into the incident. The UN announced that all UN agencies in Pakistan would be closed indefinitely until further notice.

Pakistani security officials and rescue workers survey the site of a bomb blast in Islamabad October 5, 2009. [Xinhua/Reuters]

The WFP office is located at the posh F-8 sector which was highly guarded. Local media suspected that the security measures are not efficient. When Xinhua reporters asked the security forces and special police on the scene about the cause of the blast, they did not respond and were reluctant to tell the time when they arrived at scene after the blast.

Several offices and houses of foreign diplomats are located in the sector. The private house of Pakistani President Asif Ali Zardari is located one street away from the WFP building, but the president seldom lives there nowadays.

Zardari and Pakistani Prime Minister Yousuf Raza Gilani have strongly condemned the blast at the WFP office in Islamabad and said terrorists are trying to defame Pakistan.

No group has yet claimed responsibility for the attack. But police said that it is part of the series of terrorist acts.

Pakistan's Interior Minister Rehman Malik told reporters that 56 suspects had been taken into custody for questioning.

Malik added that he did not rule out any possibility of further terrorist attacks.