It was the first attack targeting the Afghan capital's "Green Zone" since a massive truck bomb ripped through the area on May 31, killing or wounding hundreds, and prompting authorities to strengthen protection.
The Islamic State group claimed responsibility for today's attack, which follows a spate of suicide assaults by Taliban and IS insurgents on security installations and mosques in recent weeks.
A Western security source told AFP the attacker appeared to have been targeting workers leaving an Afghan defence ministry facility inside the heavily protected zone.
"The suicide attack was carried out by an underage bomber, a boy we think 13 or 15 years old, killing at least four and wounding over a dozen more civilians," interior ministry spokesman Najib Danish told AFP.
Also Read
A police spokesman said the attacker may have been as young as 12.
Defence ministry spokesman Dawlat Waziri said the bomber "made it through the first checkpoint but was stopped at the second checkpoint and detonated".
"We don't know the target but it happened a few metres from the defence ministry's foreign relations office. There were no casualties to our personnel," Waziri said.
Ghani immediately ordered an investigation into how today's attacker had been able to enter the supposedly secure area.
A Western security source told AFP he had repeatedly warned Afghan authorities about lax security at the checkpoint targeted by the bomber.
The attack struck the heart of the city's diplomatic area, where many embassies and the offices of major international organisations including the United Nations and NATO's Resolute Support mission are located.
Another witness told AFP: "A lot of people were dead and injured and there was no one to carry them away."
AFP reporters heard a loud explosion around 4:00 pm (local time) just as workers would have been leaving their offices to go home, followed by the sirens of emergency services.
Many injured people were carried from the scene of the blast, put into ambulances and police pickup trucks and driven away.
The last major assault in Kabul was on October 21 when a suicide attacker hit a busload of Afghan army trainees, killing 15.
On October 20 a suicide bomber pretending to be a worshipper blew himself up inside a Shiite mosque in the city during evening prayers, killing 56 and wounding 55.
"The Taliban and IS are trying to demonstrate a lot of potency before the winter," Vanda Felbab-Brown, a senior fellow at the Brookings Institution, told AFP.
Special truck scanners, barriers and permanent and mobile checkpoints have been rolled out across the city.
Earlier this month Kabul police stopped a lorry carrying 2,700 kilogrammes of explosives hidden under boxes of tomatoes, averting a potentially deadly blast.
"There will - and should - be a careful investigation whether the security lapse included corruption among" Afghan security forces, Felbab-Brown said.
Disclaimer: No Business Standard Journalist was involved in creation of this content