A suicide bomber dressed in military uniform struck inside a heavily fortified UN office in the Pakistani capital Islamabad on Monday, killing four Pakistanis and an Iraqi working for the food agency.
Police said they were investigating how the bomber managed to breach strict security measures and walk into the offices of the World Food Programme (WFP) and detonate about eight kilograms (17 pounds) of explosives.
There was no claim of responsibility, but blame fell on the Taliban, whose new leader Hakimullah Mehsud appeared on local television on Monday vowing "severe" new attacks to avenge the death of rebel chief Baitullah Mehsud.
His comments were apparently made on Sunday before the WFP office bombing, but he issued a chilling warning that "thousands of human lives" could be sacrificed in their insurgency to install harsh Islamic law in Pakistan.
Interior Minister Rehman Malik said the attack was carried out by Taliban extremists in revenge for a military offensive against them.
"They (Taliban militants) have prepared a strategy and there is the possibility of more such incidents in the near future," he told reporters.
There were scenes of confusion around the WFP compound in central Islamabad, with sirens blaring and smoke billowing from behind the blast walls. Wounded survivors walked amid shattered glass and blood-slicked floors.
"We were on the upper storey when the blast took place. It shook the building and shattered the windows," said one WFP employee at the scene. "We saw smoke coming out of the building, we rushed out."
UN offices across Pakistan have been closed until further notice over security concerns, United Nations spokeswoman Susan Manuel told AFP.
Bani Amin, deputy inspector general of police operations, said the explosion was caused by a suicide bomber who entered the building on foot.
"We have recovered legs and the skull of the suicide bomber. We are investigating how he managed to enter the building. There are scanners, there are cameras and strict security arrangements," Amin said.
Police and hospital officials said four Pakistanis were killed and one Iraqi, and the WFP headquarters in Rome confirmed the casualties.
"All of the victims were humanitarian heroes working on the frontlines of hunger," said WFP executive director Josette Sheeran. "This is a tragedy not just for WFP but for the whole humanitarian community and for the hungry."
UN Secretary General Ban Ki-moon called the bombing a "heinous crime" and the United States also condemned the attack.
"Senseless acts of violence against people who help feed the poor and hungry are an attack on -- on civilization itself," State Department spokesman Ian Kelly told reporters.
Interior minister Malik said that the bomber was dressed in the uniform of the paramilitary Frontier Corps -- who guard the WFP offices -- and asked to use the toilet before detonating his explosives.
Malik blamed the Taliban and said they were avenging an offensive against them which the military launched in April in northwest Swat valley, with the army now poised to begin a similar assault in the lawless tribal belt.
Taliban militants holed up in the northwest tribal belt have been blamed for a string of attacks and suicide blasts that have killed more than 2,140 people in the last two years, with 12 blasts hitting Islamabad alone.
On Monday, Taliban chief Hakimullah Mehsud appeared on local television channels and vowed revenge for the death of his predecessor Baitullah Mehsud in a US drone missile strike in the tribal belt in early August.
"We will take severe revenge for Baitullah Mehsud's killing and the continued drone strikes... both America and Pakistan will have to face the consequences," said the warlord, in comments seen by an AFP reporter.
"Our basic aim is enforcement of Islamic sharia law and if thousands of human lives need to be sacrificed, we will not hesitate," he added.
He also denied rumours of his death, which have been circulating for weeks.
The Islamabad blast was the second tragedy for the UN community here this year, with an employee from refugee agency UNHCR and another from children's agency UNICEF killed in a June suicide blast at a luxury hotel in the northwest city of Peshawar.
