Abu Louai al-Halabi, an opponent of the government of President Bashar Assad, told The New York Times 40 people may have been trapped in the ruined building. He said the dead included at least two children and two hospital employees.
The Syrian Network for Human Rights reported government forces had killed 17 people Wednesday in the Damascus suburbs.
Opposition groups said they had seized a military base in an oil-producing region in eastern Syria, the Times said. A journalist reporting from Deir Az-Zour said the rebels were in control of two of the province's three oil fields.
Rima Dali, an activist who was arrested in March, was arrested again in Damascus Wednesday. Video posted on the internet indicted Dali and three other women stood in the central market holding banners that called for an end to the fighting.
"Syria is for all of us," one sign said.