The Observatory said it had collected the names of 40 men and three women killed in Halfaya. The group also reported seeing photos of the dead bodies of 15 more unknown men.
On Sunday, it reported 60 dead.
Abdul-Rahman, the group’s head, said he could not confirm that the attack was an airstrike or that it had targeted a bakery.
Syria’s state news service blamed the attack on ‘‘an armed terrorist group’’ — its shorthand for the rebels — accusing them of filming the aftermath to ‘‘frame the Syrian army.’’
In the videos, armed rebels are clearly among those tending to the dead and wounded.
In a statement, a U.S. State Department spokesman condemned ‘‘the latest vicious attacks by the Syrian regime against civilians’’ and expressed support for Brahimi’s work.
‘‘We urge the regime to capitalize on the Joint Special Representative’s efforts in order to transition to a new government and end the brutal repression of the Syrian people,’’ the spokesman, Patrick Ventrell, said.
Associated Press writers Matthew Lee in Washington and Bassem Mroue contributed to this report.