Air strikes killed 31 civilians including 12 children in the rebel-held Eastern Ghouta region near Syria's capital Damascus on Wednesday, the Syrian Observatory for Human Rights said.
It said Syrian government air strikes and artillery hit the towns of Douma, Beit Sawa and Hammouriyeh in the insurgent-controlled suburbs. The bombing also injured 65 people, the Britain-based war monitoring group said.
The Syrian government has repeatedly said it only targets militants.
The United Nations called on Tuesday for an immediate humanitarian ceasefire in Syria of at least a month.
UN representatives noted that Eastern Ghouta, the last major rebel bastion near Damascus after almost seven years of war, had not received inter-agency aid since November.
The army and its allies have besieged Eastern Ghouta, a pocket of satellite towns and farms under the control of rebel factions, since 2013.