Children were among the reported victims as Syrian fighter jets carried out lethal airstrikes on rebel strongholds near Damascus, activists have said, amid international calls for a war crimes probe into the 22-month conflict.
Eight children were killed in an airstrike on Monday on the town of Moadamiyeh, southwest of the capital, the Syrian Observatory for Human Rights said. Five women were also reported killed in the air raid.
"The children, all members of the same clan, were aged between six months and nine years old," said Rami Abdelrahman, the head of the UK-based group.
Video footage purporting to show the aftermath of the attack showed walls blasted off apartment blocks and rubble scattered in the streets.
The Syrian government offered its own account of the blast in Moadamiyeh, saying "terrorists", its label for rebels fighting to overthrow the regime of Bashar al-Assad, fired a shell at the area, hitting a residential building and causing an undefined number of casualties.
Rebels said the strike on Moadamiyeh came amid a government offencive to push fighters from the area and the adjacent southwest suburb of Daraya.
The two suburbs were taken by rebels several weeks ago, but opposition fighters have been bogged down in clashes with government troops since then.
The Observatory said 26 children were killed across Syria on Monday, five of them in an airstrike in Aleppo province.
The UN says more than 60,000 people have been killed since the Syrian uprising began in March 2011.
At least 57 countries on Monday called on the UN Security Council to refer the Syria conflict to the International Criminal Court (ICC) for a war crimes investigation.
Switzerland sent a petition requesting the move to the 15-member council, the only body that can refer the case to the ICC but which is deeply divided over the conflict.
The signatories included many European governments as well as Libya and Tunisia, which both saw Arab Spring uprisings overthrow longstanding autocratic regimes.
The letter called on the Security Council to refer the Syria conflict for an ICC investigation "without exceptions and irrespective of the alleged perpetrators".
As Syria is not an ICC member, only a Security Council referral could start a war crimes investigation. But diplomats said the council's divide on Syria is so deep that no move by the body is now possible.
Russia and China, both veto-wielding permanent council members, have refused to sign the petition.
Meanwhile, the secretary-general of NATO said the alliance had no plans to intervene in Syria, warning that foreign intervention could have "unpredictable regional repercussions".
Anders Fogh Rasmussen told a defence conference in Sweden that Syria is more politically, religiously and ethnically complex than Libya, where NATO airstrikes in 2011 helped rebels overthrow Muammar Gaddafi.