Taliban militants claimed responsibility Tuesday for the deadly homicide bombing at the U.N. food agency's heavily fortified compound in Islamabad, saying international relief work in Pakistan was not in "the interest of Muslims."

The attack, which killed five workers for the World Food Program on Monday, pushed the U.N. to temporarily close its offices in the country and exposed the vulnerability of international relief agencies helping millions of Pakistanis, among them those affected by fighting between the government and Islamist militants.

Interior Minister Rehman Malik said the Taliban carried out the attack to avenge the Aug. 5 slaying of their leader, Baitullah Mehsud, in a U.S. drone attack.

Taliban spokesman Azam Tariq confirmed his group was behind the bombing and lashed out at foreign aid workers.

"We proudly claim responsibility for the suicide attack at the U.N. office in Islamabad. We will send more bombers for such attacks," he told The Associated Press by telephone from an undisclosed location. "The U.N. and other foreign (aid groups) are not working in the interest of Muslims. We are watching their activities. They are infidels."

He said the Taliban would not attack Muslim relief groups, but that future targets would also include Pakistani security officials, government offices and American installations.

World Food Program spokesman Amjad Jamal defended the agency's work as "totally humanitarian."

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

The suicide bomber was dressed as a security officer and was allowed to enter the World Food Program offices — apparently bypassing the normal security procedures — after asking guards outside if he could use the bathroom.

The U.N. announced it was temporarily closing its offices, but said its Pakistani partner organizations would continue distributing food, medicine and other humanitarian assistance. The world body said it would reassess the situation over the next several days.

Malik, who was visiting those injured in the bombing at a hospital, said the government had taken several of the guards outside the U.N. offices into custody for questioning as part of the investigation into the security lapse.

The United Nations had already considered itself a likely target in Pakistan. Its offices are surrounded by blast walls, while staffers are driven in bulletproof cars and not allowed to bring their families with them on assignment.

Prime Minister Yusuf Raza Gilani said those killed in the bombing Monday were serving a "noble cause."

"They will be remembered for their commendable services by the people of Pakistan," Gilani wrote in a letter to U.N. Secretary-General Bank Ki-moon, according to state-run media. "Such cowardly terrorist acts will never weaken our resolve to fight against this scourge."

The attack came a day after the new Pakistani Taliban leader met reporters close to the Afghan border, vowing more attacks in response to U.S. missile strikes. Ending speculation he had been killed, Hakimullah Mehsud denied government claims the militants were in disarray and said his fighters would repel any army offensive on their stronghold in South Waziristan.

Malik said the government was already targeting Taliban militants in South Waziristan and "if needed, further action shall also be taken at an appropriate time" against militants in other areas along the Afghan border.

In the latest fighting, airstrikes and artillery fire killed two militants and nine other people in South Waziristan, intelligence officials said on condition of anonymity. They said the army was trying to establish whether the nine unidentified victims were militants.