A suicide bomber struck outside NATO headquarters in Kabul on Saturday, killing six street children a day after the United States moved to blacklist the Haqqani faction as a terrorist organisation.

The Taliban, leading a decade-long insurgency against NATO troops and the Afghan government, swiftly claimed responsibility, saying the CIA had been the target to avenge US moves to designate as terrorists their most feared faction.

It was the deadliest attack in the fortified capital since Taliban fighters raided a nearby lakeside hotel on June 22, killing at least 18 people.

And the assault defied stepped-up Afghan security measures put in place across Kabul as national leaders commemorated 11 years since the death of Ahmad Shah Massoud, an iconic anti-Taliban commander two days before 9/11.

The bombing was also likely to renew questions about stability in Afghanistan as NATO combat troops withdraw from the country before an end-2014 deadline when they are due to transfer responsibility to their Afghan counterparts.

Separate attacks in the south killed nine other people on Saturday, including five policemen in Helmand province, where Britain's Prince Harry has deployed as a helicopter pilot, officials said.

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 Massoud.

"It was a suicide attack that killed six people and wounded five others," interior ministry spokesman Sediq Sediqqi told AFP.

Police officers confirmed the toll. Police spokesman, Hashmat Stanikzai, said the dead and wounded were all street sellers and odd jobs boys aged between 12 to 17.

Street children routinely gather outside NATO headquarters to peddle small trinkets and sweets, looking out for soldiers leaving or getting into the base.

Witnesses at the Emergency Hospital said most of the victims were teenagers or young men.

Hamid, 23, rushed to the casualty department after being told that his 15-year-old brother, Naweed, a hawker selling chewing gum and sweets, had been wounded.

"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," he said.

Gul Agha, 42, said he was searching hospitals in Kabul to find his 19-year-old son.

"Elyas my son is missing. I'm going from hospital to hospital to find him. I've been to two other hospitals already," he said.

Taliban spokesman Zabiullah Mujahid claimed responsibility, but denied police reports that the bomber was also a teenager, instead saying he was 26 years old.

"One of our mujahedeen who comes from Logar province carried out the martyrdom attack. The target was a CIA torture cell. As a result six agents were killed," he told AFP by telephone from an undisclosed location.

The Taliban routinely exaggerate claims about the success of their attacks.

He said the attack had been planned for a month, but was also a response to US moves to blacklist the Haqqani network, an Al-Qaeda-linked Taliban faction.

Founded by Jalaluddin Haqqani, a former CIA asset also close to Pakistani intelligence, it is militarily the most capable of the Taliban factions.

The network operates independently but remains loyal to Taliban leader Mullah Omar and some of the most spectacular attacks in Kabul have been attributed to its fighters.

Pakistan condemned Saturday's attack as a "cowardly act" carried out by enemies of peace and stability in the region, and pledged support in efforts to defeat terrorism in Afghanistan.

US Secretary of State Hillary Clinton said Friday she would press ahead with the Haqqani blacklisting, which will make it a crime in the United States to provide the network with any material support, and freeze any of their property or interests in the US.

The United States blamed the Haqqanis for the June hotel attack just outside Kabul, the 2011 siege on the US embassy and, in 2009, the deadliest attack on the CIA in 25 years.

Former US military chief Admiral Mike Mullen said last year the Haqqani faction had become a "veritable arm" of Pakistan's Inter-Services Intelligence. That triggered concern that the US could indirectly be branding Pakistan a terrorist state.

But US officials downplayed such fears, insisting Islamabad had been informed in advance, and stressing the move would not hamper any future peace talks with the Taliban.

US officials confirmed overnight that NATO withdrawals are on track and down from 130,000 with 117,000 foreign troops currently left in Afghanistan.