A suicide bomber has struck outside ISAF headquarters in Kabul, killing at least six people, NATO and local officials said.
The attack on Saturday, which officials say was carried out by a teenager, also wounded at least five others on a national public holiday to mourn the death anniversary of an iconic anti-Taliban leader.
The Taliban took responsibility for the attack, saying they had dispatched a bomber to target the Kabul offices of the US Central Intelligence Agency (CIA).
"One of our Mujahideen targeted an important intelligence office used for recruiting Americans and Afghans for spying," Taliban spokesman Zabihullah Mujahid said.
The bomber blew himself up near the entrance of Camp Eggers, a NATO spokeswoman said, referring to a sprawling base that is home to 2,500 coalition trainers.
Al Jazeera's Bernard Smith, reporting from Kabul, said that police reports blame a lone suicide bomber wearing an explosive vest. The teenager approached his target on a bicycle, police said.
"This attack was near the entrance of the compound - a gate where people go in and out of the headquarters. I have been through this entrance before, and you have to go through a number of barriers to get in," he reported.
The Taliban however claimed that the attacker was 28 years old, and not a child as claimed by officials.
"It was a suicide attack that killed six people and wounded five others," Sediq Sediqqi, the interior ministry's spokesperson, told AFP. Police officers confirmed the toll.
Hashmat Stanikzai, a police spokesperson, said the dead and wounded were all street sellers aged between 12 and 17.
Street children routinely gather outside NATO headquarters to peddle small trinkets and sweets, looking out for soldiers leaving or getting into the base.
Pieces of flesh and splattered blood lay on the street near the base, where small bodies were seen being lifted into ambulances, witnesses said.
"I was here when the blast occurred. I saw some wounded children on the ground. The wounded [were] transferred to emergency hospital for treatment and I heard that three of the injured children have died," said Ahmad Sameer, a witness.
Jamie Graybeal, a spokesman for the US-led international military alliance, said all coalition compounds in Kabul were currently secure. He said he was not aware of any casualties among members of the coalition.
"These are all animals, the Taliban who kill our people everyday. They told me my brother was brought to this hospital. I'm trying to get in to see him," said Hamid, a 23-year-old man searching for his 15-year-old brother who he was told was wounded in the attack.
'The Kabul green zone'
The significance of the attack is that three days before the 11th anniversary of the September 11 attacks on the United States, we have seen a young boy carrying out a suicide attack in front of the ISAF base in Kabul, about 50m from the US embassy and about 150m from the presidential palace.
This area is blocked from every side - it has four entrances, all blocked with checkpoints. It is the safest part of Afghanistan - the green zone of green zones, if you will. What is important is how a suicide bomber gets there, 11 years after the invasion. And US President Barack Obama two days ago said that US forces have killed the momentum of the Taliban. I'm not sure what they mean by that if the Taliban are able to carry out a suicide attack 50m from the entrance of the US embassy.
The Taliban are sending a message to the US, to Afghan civilians and to the [former] Northern Alliance: that they can strike anywhere in the country that they choose.
Qais Azimy is producer with Al Jazeera's Kabul Bureau
The explosion comes a day after the US designated the Haqqani network as a terrorist organisation. The group is affiliated to the Taliban and opposes the Afghan government, operating on both sides of the Afghanistan-Pakistan border.
Sediqqi speculated on his Twitter feed that Saturday's attack, just before noon, may have been carried out by the Haqqanis.
The blast reverberated through Kabul's diplomatic quarter, which is home to many Western embassies, shortly after First Vice President Mohammad Qasim Fahim finished an address to scores of dignitaries at an event mourning the death of Ahmad Shah Massoud, the North Alliance commander who was killed two days before the 9/11 attacks on the United States.
Security had been beefed up across the capital ahead of the anniversary.
It also comes just days after Afghan authorities said that they cracked down on hundreds of soldiers believed to have links to the Taliban in a bid to crush the rise of alleged insider attacks.
The announcement on Wednesday came after Anders Fogh Rasmussen, the NATO chief, expressed his concerns to President Hamid Karzai over the recent increase in the so-called green-on-blue attacks.
At least 45 NATO-led soldiers have been killed by Afghan security personnel this year - an increase from 35 killings from last year.
Saturday's attack was the deadliest in the fortified capital since Taliban fighters raided a nearby lakeside hotel on June 22, killing at least 18 people.