A Syrian activist group says government shelling has killed at least 18 people in northern Syria, while 28 rebels died near Damascus battling government forces.
The Syrian Observatory for Human Rights says mortar shells hit Sunday in the northwestern town of Ariha, which is held mostly by opposition fighters. The group says the victims included two women and a child.
Earlier, rebels and government troops engaged in fierce fighting in the town of Adra near Damascus. In addition to the rebels killed, the Observatory says a leader and some members of Syria's elite Republican Guard died in the clashes.
There is no independent confirmation of these reports.
Separately, the Observatory says Islamist fighters battling Kurdish rebels near Syria's northern border with Turkey freed 300 Kurdish civilians they had detained in exchange for the local commander of the al-Qaida-linked group.
The group says the Islamist commander in the town of Tal Abyad - known as Abu Musaab - was captured late Saturday during clashes between Kurdish fighters and a group called the Islamic State in Iraq and the Levant.
The Islamist group retaliated by rounding up civilians believed to be relatives of the Kurdish fighters to hold as bargaining chips. The fighting subsided Sunday.
Syrian Kurds have been trying to expel al-Qaida-linked 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.
Infighting between Islamists and more mainstream Syrian rebels, as well as between Kurds and Arabs, has intensified in recent weeks - part of a power struggle that is undermining efforts to topple Syrian President Bashar al-Assad.