Witnesses and officials described how the suicide bomber detonated near the Defence Ministry in the centre of Kabul just as offices closed for the day, in an attack later claimed by the Taliban.
"Twelve people, including two Afghan soldiers were killed and eight others injured," a ministry statement said, while a previous toll given by Kabul police chief Abdul Rahman Rahimi stated nine were dead and 13 wounded.
"I saw wounded civilians and army soldiers. They were begging for help but security forces did not allow common people to help them," witness Sardar Mohammad told AFP.
"The causalities, mostly, were civilians," said another man, Saleh Mohammad. "It was the time when all the people were going home."
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Ambulances converged at the site of the explosion as police and the army set up a security cordon.
Analysts have previously observed the Taliban stepping up attacks in the heart of the capital to gain leverage ahead of attempted peace negotiations with the Western-backed government in Kabul, against whom they have been fighting a bloody insurgency for more than 14 years.
No group has yet claimed responsibility for that attack, which a spokesman for the provincial governor and a police official both said targeted a tribal leader fiercely opposed to the insurgents, Haji Khan Jan.
The Taliban do not generally claim attacks with high numbers of civilian casualties, saying they only target Afghan soldiers -- "stooges" of foreign powers -- and NATO troops, considered "invaders", as well as symbols of the central government.
The blasts come amid fresh efforts by Afghanistan, China, Pakistan and the US to restart talks aimed at ending the Taliban's long and bloody insurgency in Afghanistan.
Delegates from Afghanistan, China, the US and Pakistan met in Kabul last week for a fourth round of talks aimed at forming a path back to the nascent peace process.