Syria launched a series of airstrikes targeting a stronghold of the Islamic State extremist group on Saturday, killing at least 25 people, most of whom died when one of the missiles slammed into a crowded bakery, activists said.

The eight airstrikes smashed parts of buildings, set cars alight and crushed people under rubble in the northeastern city of Raqqa, which is ruled by the extremist group, according to video of the aftermath uploaded to social media networks.

At least 16 civilians were killed, alongside nine Islamic State fighters, said the Britain-based Syrian Observatory for Human Rights. Most of the civilians were killed after at least one strike hit a bakery on a busy street, and the death toll was likely to rise, said the Observatory, which obtains its information from a network of activists on the ground.

The airstrikes were also reported by a Moscow-based activist who uses the name Abu Ibrahim and is a member of a media collective called "Raqqa is being silently slaughtered." Another group, the Raqqa Media Center, uploaded video of the aftermath which appeared to be genuine and was consistent with AP reporting of the event.

Activist Abu Ibrahim said the local morgue was packed with charred bodies, making identification difficult. He said the dead included at least eight members of one family.

Other strikes hit a government finance building that the Islamic State used as its headquarters and another building used as a jail, Abu Ibrahim said.

It has been virtually impossible for journalists to visit Raqqa, a city of some 500,000 people on the banks of the Euphrates river, since the town fell to the Islamic State group earlier this year. The group routinely abducts reporters and recently beheaded two American journalists in response to U.S. airstrikes against the militants in Iraq.

The Syrian government strikes were part of an uptick of military action against the Islamic State group since it swept into neighboring Iraq, seizing northern and western swaths of that country and declaring a proto-state straddling the border.

Syrian President Bashar Assad's government has also suffered heavy losses against the Islamic State group, which killed hundreds of soldiers and pro-government fighters in recent months as it overran oil fields and military bases. There was no immediate government comment on the airstrikes.