BEIRUT -- A Syrian government airstrike hit a bus carrying civilians in eastern Syria on Wednesday, killing at least 13 people, most of them children, opposition activists said.
But the government in the capital, Damascus, blamed the Islamic State group for the incident, describing it as "yet another massacre" committed by the extremist group.
Activists said the aircraft-fired missile struck the bus in the village of Shoula as it was traveling to Damascus from the eastern Deir el-Zour province.
The civilians on the bus were the latest victims in Syria's civil war, which is now in its fourth year and which has killed more than 190,000 people, according to the United Nations.
The conflict has pitted forces loyal to President Bashar Assad against rebels and others fighting to topple him, including increasingly powerful Islamic extremist groups. One of the militant groups, the al-Qaida-breakaway Islamic State, has taken over roughly a third of both Syria and Iraq, and declared a self-styled caliphate spanning the Iraq-Syria border.
The Local Coordination's Committees group said 13 people were killed in Wednesday's airstrike, while the Britain-based Syrian Observatory for Human Rights put the death toll at 16, including 10 children. It says the death toll was likely to rise because there were many wounded.
Syrian government airstrikes frequently target opposition-held areas of the country. Eastern Syria is controlled for the most part by the Islamic State group.
The Syrian government, in a statement on state TV, blamed the Islamic State group for the "massacre" in Shoula. It also put the death toll at 13 and said most of the victims were women and children.
Assad's forces have recently stepped up their attacks against the Islamic State group while the United States is bombing the militant group's positions in Iraq. Washington is also considering extending its airstrikes to target the group in Syria.
On Tuesday, Islamic State extremists released a video showing the beheading of a second American journalist, Steven Sotloff, and warned President Barack Obama that continuing airstrikes against the group in Iraq will be met with the killing of more Western captives.
The footage -- depicting what the U.S. called a sickening act of brutality -- was posted two weeks after the release of video showing the killing of James Foley and just days after Sotloff's mother pleaded for his life.