A suicide bomber dressed as a paramilitary soldier attacked an office of the UN World Food Programme (WFP) in the Pakistani capital yesterday, killing five staff, government and UN officials said.

Pakistan security forces have made gains this year against al Qaeda-linked Pakistani Taliban militants who have set off bombs in towns and cities aimed at security forces and government and foreign targets.

But hopes that the militants were in disarray following the killing of their leader, Baitullah Mehsud, in a US missile attack in August suffered a setback when the new Taliban chief surfaced to deny differences in his ranks.

The suicide bomber was disguised as a paramilitary soldier and got into the WFP compound after asking a guard at the gate if he could use a toilet, a government minister said.

A WFP spokesman, Amjad Jamal, said five members of staff had been killed, four Pakistanis and an Iraqi. Two of the Pakistanis were women.

"I went to my office on the first floor and as I sat on my chair there was a huge blast," WFP official Arshad Jadoon said outside the tightly guarded office in a residential area of Islamabad.

"All of a sudden, a smoke cloud enveloped the building and we came out where wounded people were lying," he said.

UN Secretary-General Ban Ki-moon said the attack was a heinous crime.

The United Nations temporarily closed its office in Pakistan after the blast for security reasons, a UN spokesman said.

Two foreign UN workers were killed in a suicide car bomb attack on a hotel in the northwestern city of Peshawar in June.

Yesterday's blast led to a brief spate of selling at Pakistan's main stock market but the market recovered to end 0.35 per cent higher at 9,487.95 points.

Interior Minister Rehman Malik had been saying the back of the Pakistani Taliban has been broken but the militants have struck back with several bombs in recent days as the army prepares to launch an offensive on their main bastion in the South Waziristan region on the Afghan border.