Government troops fired mortar rounds that slammed into a main market in a town in northern Syria on Sunday, killing at least 20 civilians, activist groups said.
The mortar shells struck the town of Ariha, which is held mostly by opposition fighters, a few hours ahead of iftar, the meal that breaks the dawn-to-dusk fast during the Muslim holy month of Ramadan.
The UK-based Syrian Observatory for Human Rights and the Local Coordination Committees, two opposition groups tracking the violence in Syria, said at least 20 people were killed including two children and two women. It was not immediately clear what triggered the shelling.
Also today, state media said government forces killed nearly 50 rebels in an ambush near Damascus.
Separately, Kurdish rebels freed the local commander of an al-Qaeda-linked group in a town near Syria's northern border with Turkey in return for 300 Kurdish civilians detained by the group, as part of an agreement to end rebel infighting that erupted a day earlier in the region.
The commander in the town of Tal Abyad, who is known as Abu Musaab, was captured during intense fighting between the Islamic State in Iraq and the Levant and Kurdish militants late yesterday, the Observatory said.
The Islamic fighters retaliated by rounding up civilians believed to be relatives of the Kurdish fighters to hold as bargaining chips.
Infighting between al-Qaeda militants and more mainstream Syrian rebels, as well as between Kurds and Arabs, has grown more common in Syria in recent weeks, part of a power struggle that is undermining their efforts to topple President Bashar Assad.
Kurdish gunmen have been fighting to expel al-Qaida militants, many of whom are foreign fighters, from the northeastern province of Hassakeh over the past week. More than 60 fighters have been killed from both sides, according to activists.
Last evening, the fighting spread to Tal Abyad, which is located in neighbouring Raqqa province near the Turkish border.
The inter-rebel clashes, along with the efforts by extremist foreign fighters to impose their strict interpretation of Islam in areas they control, are chipping away at the opposition's popularity at a time when the Assad regime is making significant advances on the ground.
In recent weeks, Assad's troops have seized the momentum in the civil war, now in its third year. His forces have been on offensive against rebels on several fronts, including in the north.