At least 13 people have been killed in a suicide car bombing in Kabul, the Afghan defence ministry has said.
The bomber attacked a bus carrying Afghan soldiers during morning rush hour in the south-west of the capital.
The defence ministry said six soldiers and seven civilians were killed and a dozen other people were injured.
It is the third suicide bombing in the city in a week. More than 3,000 people have been killed this year as Afghan and foreign forces battle the Taleban.
Wednesday's blast came a day after a similar attack on a Nato convoy close to Kabul airport injured at least 22 Afghans.
The number of dead in the latest attack is expected to rise as several of the wounded have been severely injured.
A spokesman for the Taleban said the group carried out the attack.
Witnesses said the bomb was in a small car that drew up close to the bus before exploding early on Wednesday.
KEY BOMB ATTACKS SINCE 2001
6 November 2007
At least 70 die after suicide bombing in Baghlan province
29 Sept 2007
Suicide attack on Kabul bus kills 30
10 Sept 2007
At least 20 dead in two suicide bombs in Gereshk, Helmand province
17 June 2007
At least 35 dead in bomb attack on Afghan police bus in Kabul
3 Aug 2006
Car bomb kills at least 21 in Kandahar province
16 Jan 2006
Suicide bomb kills at least 20 people in Spin Boldak, Kandahar province
1 June 2005
At least 20 people killed in bomb attack on mosque in Kandahar city
The blast also caused gas bottles in a nearby shop to explode, witnesses said.
The bus carrying the soldiers was destroyed in the attack.
"It was a big explosion and sent fire into the sky," said Akbari Sarwar, a journalist who witnessed the bombing, AFP news agency reports.
He described a scene of carnage.
The BBC's David Loyn in Kabul says buses carrying soldiers and police have increasingly been the target of suicide bombers in recent weeks.
In September, 30 Afghan soldiers were killed when their bus was attacked in Kabul.
US Defence Secretary Robert Gates has just been in Kabul assessing an increase in violence in Afghanistan.
He said the increase was partially due to an offensive against Taleban militants by international troops from the Nato-led International Security Assistance Force (Isaf).
Afghan military officials say they need more training and equipment to fight the growing Taleban insurgency.