A roadside bomb killed four people in a bus carrying Afghan government staff in Kabul on Thursday, a day after nine members of the security forces died when their helicopter was shot down, officials said.

No one has yet claimed responsibility for the roadside blast though the government has blamed Taliban insurgents for recent attacks targeting government employees, civil society figures and journalists.

The Taliban has denied involvement in the campaign.

The bomb hit a bus rented by the Afghan Ministry of Information and Technology to transport employees, said Abdul Samad Hamid Poya, a ministry adviser.

The bombing comes on the same day the Afghan government, Taliban and key countries, including the United States and Russia, gather in Moscow to push for a reduction in violence to propel the Afghan peace process forward.

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 take-off in central Maidan Wardak province.

It was not clear who fired the rocket and there have been no claims of responsibility for the crash late on Wednesday.

The defense 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.