Seven children were among at least 13 civilians killed in an air raid on Raqa, the only provincial capital in rebel hands, according to the Syrian Observatory for Human Rights.
It said the raid was apparently aimed at positions of the jihadist Islamic State of Iraq and the Levant (ISIS) which largely controls the city.
ISIS has been the dominant force in the city since its capture by rebels in March. Residents have held several protests against the policies of ISIS which follows an extremist line of Islam, according to the Observatory.
In the coastal Latakia province of northwest Syria, at least 20 people were killed in several air strikes on the Sunni rebel town of Salma in the Jabal al-Akrad area, the Observatory said.
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At least six of those killed were Syrian rebel fighters, while four were foreign volunteers, said Rami Abdel Rahman, who heads the Observatory.
Latakia province is a stronghold of the Alawite minority of President Bashar al-Assad, apart from rebel-held pockets.
Islamist rebel forces have captured about 10 Alawite villages in the Jabal al-Akrad, a mountainous area of the province. The army has hit back, sparking fierce fighting that has left dozens dead on both sides, according to the Observatory.
In Aleppo province, further east, government troops stormed a village overnight, killing 12 people, the Observatory said.
In the eastern city of Deir Ezzor, Al-Nusra Front jihadists and other rebel fighters seized control of the offices of Syria's ruling Baath party in the Howeika district, sparking regime bombardment, the Observatory said.
ISIS fighters have clashed for the past month with Kurdish militia forces in the Raqa region, triggering a warning on Saturday from Iraqi Kurdish leader Massud Barzani of a cross-border intervention to protect fellow Kurds.
More than 100,000 have been killed in the past 29 months of conflict.