Air strikes killed 23 people at a tourism spot in northwest Syria on Friday, the Syrian Observatory for Human Rights said, on the last day of a ceasefire announced by the Syrian army.
The air strikes hit a riverside area in the town of Darkush, near the Turkish border, in western Idlib province. Idlib province and city are strongholds of rebel groups including the al Qaeda-linked Nusra Front.
The dead and injured had come from towns around the province to enjoy the Eid holiday weekend, witnesses and the Observatory said.
The death toll, which included 10 women and two children, is likely to rise due to the number of severely injured people, the Observatory said.
A 72-hour ceasefire was announced by the Syrian army on Wednesday, but rebels and the Observatory said there had been little let-up in the violence.
On Thursday, Syrian government forces took a step towards completely encircling rebel-held parts of Aleppo, capturing ground overlooking the only road into the opposition half of the city and effectively putting those areas under siege.
Syrian and Russian jets carry out air strikes across Syria but it is not known which carried out the attack in Idlib on Friday.
Russia sent warplanes to Syria last year to support President Bashar al-Assad against rebels seeking to end his rule, and have supported Syrian government forces in a separate fight against Islamic State further east.
Fighting in Syria’s five-year-old civil war has intensified since a February ceasefire deal unravelled.