BEIRUT — Syrian opposition activist groups said airstrikes Sunday killed at least 17 people just south of the northern city of Raqqa, the de facto Islamic State capital.

The Britain-based Syrian Observatory for Human Rights said 18 people were killed in the airstrikes on the road between the villages of Ratla and Kasrat. It said the raids hit buses, adding that the identities of the dead are not known.

The activist-operated Raqqa is Being Slaughtered Silently said 17 civilians were killed in the airstrike.

Both groups blamed the US-led coalition, which has been carrying out airstrikes in Syria against the Islamic State since September 2014.

US-backed Syrian fighters have been marching toward Raqqa for weeks under the cover of coalition airstrikes.

In Afghanistan on Saturday, at least 18 people, mostly civilians, were killed when a suicide car bomber targeted a convoy of provincial security forces in eastern Afghanistan, an Interior Ministry official said.

Najib Danish, the ministry’s deputy spokesman, said the target of the bomber was a group of guards providing security for US forces in Khost province, but most of the victims were civilians.

Zabihullah Mujahid, a spokesman for Taliban, claimed responsibility for the attack in a media statement. Taliban insurgents have increased their attacks against Afghan security forces since announcing their spring offensive last month.

The security forces were attacked near the province’s main bus station, Danish said. ‘‘Six other civilians including two small children were wounded in the attack,’’ he said.

The attack, which took place on a road between the bus station and the provincial football stadium, damaged or destroyed about a dozen shops and several vehicles.

The attack came on the first day of Ramadan, the Muslim holy month.

Mubariz Zadran, spokesman for the provincial governor, said the casualty number could rise because some of the wounded were in critical condition.

Associated Press