The explosion happened early in the afternoon
The Pakistani Taliban have said they launched Monday's attack on the UN World Food Programme offices in the capital Islamabad, killing five people.
A Taliban spokesman said the international agency had been targeted because its work was "not in the interest of Muslims".
A suicide bomber dressed in military uniform blew himself up in the office.
The Taliban had promised revenge for the killing of their leader Baitullah Mehsud in a US drone strike in August.
The Taliban spokesman who called himself Azam Tariq said that the UN and foreign aid agencies were "infidels" promoting an American agenda.
The group has been behind a series of recent attacks.
Last week, at least 16 people died in two suicide car bomb attacks in north-western Pakistan.
Interior Minister Rehman Malik said an investigation had been launched into how security officials allowed the bomber, who was wearing a uniform of the paramilitary Frontier Corps, to enter the WFP offices.
The attack came as Britain's defence and home ministers visited Islamabad for talks with Pakistani officials on the role Pakistan plays in combating terror.