Media playback is unsupported on your device Media caption The BBC's Jonathan Beale in Kabul says he heard an explosion followed by sirens

A teenage suicide bomber has killed at least six people near the headquarter of the Nato-led international coalition (Isaf) in Kabul.

A number of children are among the dead. There were no reports of casualties among Isaf troops.

The police said the attacker was a boy, 14, on a motorbike, who detonated a bomb near an entrance to the HQ.

Kabul security has been tightened as supporters of an anti-Taliban warlord mark 11 years since his assassination.

Ahmad Shah Massoud - a hero of the 1980s war against Soviet occupiers, and later of opposition to the Taliban - was killed by al-Qaeda suicide bombers on 9 September 2001.

Following Saturday's explosion, the Isaf HQ, home to some 2,500 personnel, was placed "on lockdown", the Isaf spokeswoman said.

'Child hawkers'

Child street hawkers are believed to have been caught in the blast and witnesses quoted by Reuters said small bodies could be seen being carried to ambulances.

A police official speaking on condition of anonymity told AFP news agency: "Most of the victims are young children who gather around Isaf to sell small items to soldiers leaving or getting into the base."

The US embassy, the Italian embassy and the presidential palace are also located near the site of the attack.

The Taliban have claimed they were behind the attack, but say it was carried out by a man in his 20s, targeting a building used by the CIA to train Afghan spies.

But the BBC Jonathan Beale, in Kabul, says there is also speculation that it could have been the work of another insurgent group - the Haqqani network - which carried out a series of coordinated attacks in the city earlier this year.

Scores of dignitaries were attending commemorations of Massoud's death in Kabul on Saturday, which is a national public holiday in his honour.