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 Isil 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 travelling to Damascus from the eastern Deir Al 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 Al Assad against rebels and others fighting to topple him, including increasingly powerful Islamist extremist groups. One of the militant groups, the Al Qaida-breakaway Isil, has taken over roughly a third of both Syria and Iraq, and declared a self-styled caliphate stretching across the Iraq-Syria border.

The Local Coordination’s Committees group said 13 people were killed in Wednesday’s air strike, 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 air strikes frequently target opposition-held areas of the country. Eastern Syria is controlled for the most part by the Isil group.

The Syrian government, in a statement on state TV, blamed the Isil group for the “massacre” in Shoula. It also put the death toll at 13 and said most of the victims were women and children.

Al Assad’s forces have recently stepped up their attacks against the Isil 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.

Meanwhile, a leading international watchdog said Isil militants carried out a mass killing of hundreds of Iraqi soldiers captured when the extremists overran a military base north of Baghdad in June.

The incident at Camp Speicher, an air base that previously served as a US military facility, was one of the worst atrocities perpetrated by the Isil in its lightning offensive that seized large swaths of northern and western Iraq.

According to Human Rights Watch, new evidence indicates the Isil fighters killed between 560 and 770 men captured at Camp Speicher, near the city of Tikrit — a figure several times higher than what was initially reported.

“These are horrific and massive abuses, atrocities by the Islamic State [of Iraq and the Levant], and on a scale that clearly rises to the crimes against humanity,” Fred Abrahams, special HRW adviser, told reporters in the northern city of Arbil on Wednesday.