BEIRUT -- Syrian government airstrikes, including one on a crowded sheep market, killed at least 11 people on Thursday in a northern town controlled by the Islamic State group, activists said.

But the government has long deployed its heaviest air power against mainstream and Islamist rebel groups, and on Thursday it launched 12 airstrikes on the town of Douma, east of Damascus, that smashed buildings and killed at least nine people, activists said. The area is mainly controlled by rebel groups, and Islamic State militants have little if any presence there.

After largely ignoring the rise of the al-Qaida splinter group over the past year, President Bashar Assad's air force has begun to target Islamic State militants more regularly since the extremist faction seized control of much of northern Syria and a large part of neighbouring Iraq in June.

Two activist groups, the Aleppo Media Center and the Syrian Observatory for Human Rights, said Thursday's air raids struck the town of al-Bab in Aleppo province.

The Britain-based Observatory said at least 11 people were killed and more than 17 wounded. The AMC put the death toll at 19 and said another 20 people were wounded.

It was not possible to reconcile the difference in casualty figures, but numbers frequently vary in the chaotic aftermath immediately following attacks.

The strike on the market left scenes of carnage. In one amateur video posted online, civil defence workers scamper around charred hulks of bombed-out cars and trucks. Three bodies, their limbs at odd angles, lie in a pool of blood. The fluffy coat of crushed sheep can be seen in the back of one half-smashed truck.

The video appeared genuine and corresponded to other Associated Press reporting.

Al-Bab is one of the Islamic State group's strongholds in Aleppo province, where the extremists have been chipping away at territory long-held by Syria's mainstream rebels.

The beleaguered mainstream armed opposition has largely been on the losing end of a bloody nine-month battle with the Islamic State group across northern Syria.

But those rebels, who also are fighting Assad, received a long-awaited boost on Wednesday when President Barack Obama said he would authorize U.S. airstrikes inside Syria for the first time. Such strikes, along with an expanded aerial campaign in Iraq, are part of what Obama called "a steady, relentless effort" to root out and ultimately defeat Islamic State extremists.

The airstrikes on Douma underscored the threat mainstream rebels continue to face from Assad's forces. The Observatory said the airstrikes killed at least nine people, but an activist based in Douma, Hassan Taqulden, said dozens more were believed to have been killed in the strike, with many bodies crushed beyond recognition.

Videos uploaded to YouTube showed a bloodied boy being carried away, his arm blown off. An elderly man lay in a pool of blood, struggling to breathe. One youth held an unconscious man, screaming, "Save him!" Another video showed the dead bodies of children on the ground as buildings burned nearby. The videos corresponded to Associated Press reporting of the event.

Syrian state-run news said the army targeted "terrorists" in Douma, killing and wounding them. The government refers to all armed rebels as terrorists.

Rebels based in Douma and other areas on the outskirts of Damascus have shelled government-controlled neighbourhoods in the capital, bringing fierce government reprisals that activists say amount to collective punishment.