The town of Hammuriyeh in Eastern Ghouta was hit with airstrikes on Tuesday morning, according to the Britain-based Syrian Observatory for Human Rights. The government delegation will arrive on Wednesday to attend the talks, state news agency SANA reported.
The locally run Ghouta Media Center put the number of civilians killed at 23. The observatory said it determines aircraft used in air raids by location, flight patterns and munitions used.
At least 400,000 people have been killed in Syria's civil war that followed a violent crackdown on demonstrations in 2011 against the rule of the family of President Bashar Assad.
Five people, including a child, were also killed in Medeira and two others in Douma, the sources said.
Multiple attempts to end the conflict have failed.
The implementation of the zone, agreed earlier this year between government allies Russian Federation and Iran, and rebel backer Turkey, initially reduced the violence in Eastern Ghouta, AFP said. However, the government's siege has caused a humanitarian crisis in the area, causing severe medical and food shortages.
Sunday's deaths come despite Eastern Ghouta falling within a so-called "de-escalation zone" that was put in place by a deal brokered between Assad's allies Russian Federation and Iran as well as rebel-backer Turkey. Residents are so short of food that they are eating trash, fainting from hunger and forcing their children to eat on alternate days, the U.N. World Food Programme said in a report last week.
United Nations -backed peace talks in Geneva are set to begin on Tuesday.
Médecins Sans Frontières, the worldwide medical humanitarian organization, said on Monday hundreds of people had been wounded in intense bombing and shelling of the Eastern Ghouta in the last two weeks.