The attacker struck at a time when workers were arriving at their offices. It comes a week after militants stormed a National Directorate of Security (NDS) training centre in the Afghan capital.
Interior ministry spokesman Najib Danish told AFP six civilians in a car were killed when the attacker detonated himself.
"Six people were martyred and three others were wounded," Danish said.
"They were hit when they were passing the area in their Toyota sedan vehicle. We still do not know the target of the attack but it happened on the main road."
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There was no immediate claim of responsibility for the latest attack in Kabul which in recent months has become one of the deadliest places in the war-torn country for civilians.
An AFP reporter at the blast site said the attack happened outside the main entrance to an NDS compound. Security forces have swarmed the area, closing off the main road leading to the building.
Ambulances were seen leaving the scene, apparently taking casualties to hospitals in the city.
"Our initial information shows a blast took place near an intelligence headquarters in Shash Darak neighbourhood of Kabul," deputy interior ministry spokesman Nasrat Rahimi told AFP.
It has scaled up its attacks in Kabul, including those on the country's Shiite minority.
The resurgent Taliban and increasingly IS are both stepping up their assaults on security installations and mosques.
On Friday, a suicide bomber drove an explosives-packed Humvee into a police compound in the southern province of Kandahar, killing at least six officers and destroying a building.
The Taliban claimed responsibility for the pre-dawn ambush.
Afghan forces, already beset by desertions and corruption, have seen casualties soar to what a US watchdog has described as "shockingly high" levels since NATO forces officially ended their combat mission in 2014 and began a training and support role.
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