Police and security forces survey the World Food Program building in Islamabad after a suicide bomb attack on Monday. (Reuters)

Taliban militants have claimed responsibility for the bombing of a United Nations office in Islamabad.

The suicide bombing at the World Food Program building on Monday killed at least five people and wounded several others, prompting the UN to temporarily close its offices throughout Pakistan.

The attack came a day after the new leader of the Taliban in Pakistan, Hakimullah Mehsud, vowed fresh assaults against Pakistan and the United States in retaliation for the increased number of drone-airplane attacks in the tribal areas on the border with Afghanistan.

Taliban spokesman Azam Tariq said Tuesday the food agency was the target of the bombing because international relief work in Pakistan is not in "the interest of Muslims."

"We proudly claim the responsibility for the suicide attack at the UN office in Islamabad. We will send more bombers for such attacks," Tariq told The Associated Press. "The UN and other foreign [aid groups] are not working for the interest of Muslims. We are watching their activities. They are infidels."

The Taliban will not target Muslim relief groups, he added.

'Totally humanitarian'

World Food Program spokesman Amjad Jamal said the agency's work was "totally humanitarian."

The World Food Program is distributing food to poor Pakistanis, including those in the northwest who have been displaced or affected by a recent Pakistani army offensive against militants in the Swat Valley.

"We provide food. Our food is for the vulnerable groups, the poor groups who cannot afford one meal a day," Jamal said.

Though the offices will be closed, the delivery of food aid to refugees will not be halted, he said.

UN offices remained closed in the country on Tuesday. Officials said the organization will be assessing the situation over the next several days and will continue to deliver assistance through Pakistani partner agencies during that time.

The United Nations considers itself a major target in Pakistan. Many of its offices are surrounded by high blast walls. Its staff members are driven in bulletproof cars and not allowed to bring their families with them on assignment in the country.

Best-protected UN building in Pakistan

The World Food Program compound, which employs more than 70 people, is surrounded by square metal cages filled with sand and small stones used to protect against blasts and projectiles.

"This was one of the best-protected UN centres in all of Pakistan," said UN spokeswoman Michele Montas at the world body's headquarters in New York. "We were really quite heavily guarded, at least at that compound. How that person got in — that is still being investigated, and we're trying to find out from surveillance cameras."

The suicide bomber, who was believed to be in his 20s, was disguised as a paramilitary officer and was reportedly allowed to enter the building after asking permission from a security guard to use a restroom.

Security camera footage broadcast on local TV shows the bomber walking through a door into what appears to be the main building carrying a long cylindrical object before detonating about eight kilograms of explosives in the lobby.

With files from The Associated Press