PESHAWAR, PAKISTAN - The Pakistani Taliban on Tuesday claimed responsibility for a blast at a United Nations office in the capital which left five aid workers dead, a spokesman for the insurgents told AFP.

"This attack was launched by us - we claim responsibility," Taliban spokesman Azam Tariq told AFP by telephone from an undisclosed location.

Four Pakistanis and an Iraqi were killed Monday when a man dressed in military uniform breached strict security measures and detonated explosives in the heavily-fortified office of the World Food Programme (WFP) in Islamabad.

"The WFP is promoting the US agenda. They are silent on massacres and do not comment on killings in Waziristan and other areas," Tariq said, referring to a lawless tribal district where the US has launched missile strikes.

Pakistan's military has said it is readying for an assault on the northwest tribal belt near Afghanistan, a bastion of the Tehreek-e-Taliban Pakistan movement and an alleged hideout and training ground for Al-Qaeda fighters.

"Pakistan has been threatening a military operation. We also reserve the right to retaliate. We will give them a befitting response," Tariq said.

Interior Minister Rehman Malik has already blamed the blast on Taliban militants, saying they were avenging a military push against them in northwest Swat valley launched in late April which left more than 2,000 militants dead.

The Taliban have also vowed attacks to avenge an August 5 US drone missile strike in South Waziristan that killed insurgent commander Baitullah Mehsud.