A suicide bomber dressed in military uniform has attacked the UN World Food Programme offices in Pakistan's capital Islamabad, killing five people.
Pakistan's interior minister said an investigation had begun into security lapses after guards had allowed the man into the compound to go to the toilet.
Four of the dead are Pakistanis, the fifth is an Iraqi. The bomber died too.
It is unclear who is responsible but suspicion will fall on the Pakistani Taliban, correspondents say.
They promised revenge for the killing of their leader Baitullah Mehsud in a US drone strike in August and have been behind a series of recent attacks.
Last week, at least 16 people died in two suicide car bomb attacks in north-western Pakistan.
'No to terrorists'
Local television footage showed smoke rising from the heavily fortified UN building, shortly after the early afternoon blast in its reception area.
The explosion happened early in the afternoon
A number of injured people are being treated in hospital.
The BBC's Orla Guerin in Islamabad says heavily armed anti-terror police quickly ringed the compound and sniffer dogs were brought in.
For Pakistan this was an unwelcome reminder that their capital remains vulnerable, our correspondent says. It is further proof that the militants can still strike in spite of increased security precautions and ongoing army operations.
Interior Minister Rehman Malik said an investigation had been launched into the security officials who had allowed the bomber, who was wearing a uniform of the paramilitary Frontier Corps, to enter.
Mr Malik said the attack would not "slacken the resolve" of Pakistan's efforts to counter the Taliban.
He said: "The operations that we carried out against them in Swat, North Waziristan and South Waziristan have broken their back. They are like a wounded snake."
Mr Malik added: "I want to make it clear to the terrorists that the entire nation is united, and the entire nation says no to Taliban, no to oppressors, no to terrorists, no to extremists."
Earlier, a WFP employee, Sajjad Anwar, said about 100 people were working in the compound at the time.
"Walls of the building have cracked because of the intensity of the blast," he said.
"I don't know how this could have happened. We have private security as well as government-provided police."
The WFP said five of its staff members had been confirmed dead.
They included two Pakistani finance assistants and an Iraqi information and communication technology officer.
United Nations Secretary General Ban Ki-moon condemned the attack.
"This is a heinous crime committed against those who have been working tirelessly to assist the poor and vulnerable," he said in Geneva.
The attack came as Britain's defence and home ministers visited Islamabad for talks with Pakistani officials on the role Pakistan plays in combating terror in the UK.
A British embassy spokesman said: "I'm not prepared to say where they are staying, but it is safe to say they were not affected."
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