Four people were killed and several others injured after a roadside bomb ripped through a bus in Kabul on Thursday.
The blast hit a bus hired by the Afghan Ministry of Information and Technology and was carrying some of its employees, officials said.
It was not immediately clear if any of the deceased were government staff.
The Taliban has denied responsibility but government officials, civil society figures and journalists have blamed the extremist group for other recent attacks.
Thursday's killings came on the day the Afghan government, Taliban and key countries – including the United States and Russia – gathered in Moscow to push for a reduction in violence to propel the Afghan peace process forward.
On Wednesday, nine members of the security forces died when their helicopter was shot down, in another suspected Taliban attack.
Afghanistan's president, Ashraf Ghani, told reporters on Thursday that the helicopter had been shot down and the perpetrators would be arrested and punished.
"Today was a very hard day," he said.
Two sources said the helicopter was hit by a rocket during takeoff in central Maidan Wardak province.
It was not clear who fired the rocket and there were no immediate claims of responsibility.
The defence ministry said it was investigating the crash, which killed the helicopter's crew members as well as members of the special forces on board.
An air force source said the helicopter had been on a supply mission that included transporting the body of a soldier and wounded members of the military.
Additional reporting by Reuters
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