BEIRUT (AP) — Air strikes in Syria targeted the Islamic State's de facto capita of Raqqa on Wednesday, killing at least 18 civilians, including two children, according to activists. The barrage, blamed on the Russian and Syrian air force, came as government forces sought to recover from losses suffered in a stalled offensive on the extremist-held city.

Activists from the group known as Raqqa Is Being Slaughtered Silently, which relies on residents to smuggle news out of IS-held territory, said the air raids also wounded 28 people.

The Britain-based Syrian Observatory for Human Rights put the toll at 25 civilians, including six children, but different casualty tolls are common in reporting from Syria's civil war, now in its sixth year.

Both activist groups blamed the Russian and Syrian air forces for Wednesday's airstrikes, saying Damascus and its ally Moscow were pummeling the Islamic State extremists amid frustrations over government losses earlier this week.

There was no immediate comment from the Syrian government. The U.S.-led coalition has also been bombing Raqqa, the IS bastion that is also the seat of the Sunni militant group's self-proclaimed caliphate. It's also the capital of a Syrian province with the same name.

Raqqa Is Being Slaughtered Silently said at least one of the airstrikes targeted a neighborhood popular among "foreign fighters" — militants who have migrated to Syria to fight with the extremist IS group.

The Islamic State group retook large swaths of the Raqqa province from government forces on Tuesday, just two days after Syrian troops managed briefly to seize Thawra, an IS-held oil field, and threatened to retake the Tabqa air base from the militants.

The government's highly-publicized campaign to retake Raqqa began on June 2. On Sunday, Syrian government forces advanced to within six miles (10 kilometers) of the Tabqa base. The base, located 45 kilometers (28 miles) from Raqqa, holds strategic and symbolic value in the government campaign to unseat the extremists from the city of Raqqa.

It was the last position held by government forces in Raqqa province before IS militants overran it in August 2014, killing scores of detained Syrian soldiers in a massacre the militants documented on video. Raqqa itself became the militants' first captive city.