The Taleban say they have carried out a bomb attack on a minibus in the southern Afghan city of Kandahar which killed at least 10 people.
The bus was carrying Afghan workers to a coalition military base
The vehicle was carrying Afghan labourers on their way to work at a coalition military base in the city.
Correspondents say it was the first such major attack on Afghan civilians working with US forces in the country.
Police say the bomb was possibly hidden on the bus, and detonated during morning rush hour.
Attacks blamed on Taleban have risen in southern and eastern Afghanistan this year, with hundreds of deaths.
A Taleban spokesman claimed responsibility for the latest incident in a telephone call to a local news agency. He said that the Taleban had warned Afghan nationals not to work with US forces.
President Karzai said "enemies of Afghanistan deliberately targeted innocent civilians".
Meanwhile officials say that the second most senior judge in Ghazni province, Jan Mohammad, was killed on Wednesday after he was abducted along with his young son.
Officials say that the judge was shot dead outside his house, and that the whereabouts of his son - who was taken away in the car of the attackers - is unknown.
The Governor of Ghazni province, Sher Alam Ibrahimi, described the incident as "the work of enemies of peace and stability".
Officials were also swift to condemn Thursday's bomb blast in Kandahar in which at least 15 people were injured.
"It is a bomb explosion and it seems that the bomb was inside the minibus," Colonel Shir Shah told Reuters news agency.
Kandahar bakery owner Amidullah was working in his shop when the bus exploded outside.
"I heard an explosion and one of my customers and one of my workers was injured," he told the Associated Press.
"When I came out of the shop, I saw the bus totally destroyed on the ground. I saw people dead and wounded lying on the ground."
US officials said the bombing marked a change in Taleban tactics.
"It's the first time Afghans working here have been deliberately targeted by the Taleban," coalition spokesman Maj Quentin Innis said.
"They are clearly a non-military target being targeted. That's a shift in tactics for the Taleban."
There has been a surge in violence in southern Afghanistan, coinciding with the arrival of thousands of international troops trying to help the government bring security to areas where it has little control.
As well as fighting between coalition forces and Taleban rebels, there have been many roadside bombings, suicide attacks and assassinations of local government officials.
Thursday's blast came two days after two US troops were killed in clashes with the Taleban, and less than a week after the death of UK soldier.
More than 30 foreign soldiers have been killed in action in Afghanistan this year, most of them American.