BEIRUT — Airstrikes in Syria targeting the Islamic State’s de facto capita of Raqqa killed at least 18 civilians, including two children, activists said Wednesday. The barrage, blamed on the Russian and Syrian air force, came as government forces sought to recover from losses in a stalled offensive on the extremist-held city.

Meanwhile, Syrian President Bashar Assad designated a new prime minister to form the government following April parliamentary elections. The vote, held only in government-controlled areas, was dismissed by the opposition and much of the international community as a rubber-stamp election for the president.

The strikes on Raqqa, which took place on Tuesday evening, came after government forces suffered setbacks fighting IS in the north.

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 killed 18 and 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 the raids, saying Damascus and its ally Moscow were pummeling the Islamic State extremists amid frustrations over government losses earlier this week.

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 is also the capital of a Syrian province with the same name.

There was no immediate comment from the Syrian government.