Syrian air strikes against the northern jihadist bastion of Raqa on Saturday killed 53 people, a monitor said, updating an earlier toll and adding that at least 31 of the dead were civilians.
"We have documented the deaths of 31 civilians, among them five women and three children, in Raqa and its surroundings," said Syrian Observatory for Human Rights director Rami Abdel Rahman.
Another 15 jihadists were confirmed dead in a string of eight air strikes, as well as seven other unidentified people.
Eight of the civilian victims were from one family, said Abdel Rahman.
The strikes come amid a stepped-up campaign by President Bashar al-Assad's regime targeting IS positions in northern and eastern Syria.
Activists say such air raids often kill many civilians as well as jihadists.
Similar strikes on Tuesday in the eastern province of Deir Ezzor killed 16 people, including 10 children.
The targets on Saturday included a building housing an IS Islamic court and a training camp. Twenty-four of the dead were killed when a bakery was hit.
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The Islamic State group first emerged in Syria's war in late spring 2013, when it was named the Islamic State of Iraq and the Levant (ISIL).
Its horrific abuses, including near-daily public executions, have made it widely feared in areas it controls, but activists say people mostly stay silent for fear of reprisals.
In a separate development, the Observatory raised its death toll from a Friday air strike targeting a Syrian rebel-held area of Aleppo city in the north to 15.
Insurgent areas of Aleppo have come under massive aerial bombardment since December, killing hundreds of people and prompting tens of thousands to flee to safety.
The regime has kept up its offensive despite a UN Security Council resolution in February condemning such strikes.
And east of Damascus, regime troops backed by Lebanese Hezbollah fighters made fresh gains against rebel forces by taking the village of Hteitet Jarash, the Observatory said.
The village is near Mleiha, a former rebel bastion that fell to government control in mid-August after several months of fierce fighting and near-daily bombardment.
Hteitet Jarash is in the Eastern Ghouta area, which the army has besieged for more than a year.
Syria's war began in March 2011 as a peaceful protest movement demanding Assad's ouster, but morphed into a brutal war after the regime unleashed a massive crackdown against dissent.
The United Nations says at least 191,000 people have been killed in the more than three-year conflict.