The Syrian army fired a missile into the last rebel-held neighbourhood of the central city of Homs killing 17 people, most of them civilians, a monitoring group said on Sunday.
Four children and four women were among the dead in Saturday's strike, the Syrian Observatory for Human Right said.
Nearly all of Homs has been under government control since rebel fighters pulled out of the historic heart of the city in May last year under a UN-brokered deal to end a devastating two-year siege.
There have been repeated efforts to organise a similar deal for the Waer neighbourhood where more than 100,000 civilians live under rebel control surrounded by government forces.
A number of temporary ceasefires have been arranged to allow in aid but a lasting truce has proved elusive, with each side accusing the other of hobbling the negotiations.
The missile strike on the enclave came as a UN-brokered truce deal to evacuate 10,000 civilians from two pro-government villages in northwestern Syria under siege by rebels ran into trouble on Saturday.
Protesters in neighbouring rebel-held territory in Idlib province blocked roads, preventing the planned evacuation by the Red Crescent from getting under way.
The rebels, who have laid siege to Fuaa and Kafraya for two months, agreed to their evacuation in return for safe passage for some 600 rebel fighters who have been surrounded by pro-government forces in the town of Zabadani near the Lebanese border.
Syria's third largest city, Homs was one of the first places where rebels took up arms against President Bashar al-Assad's rule in 2011.
But after years of fighting which killed tens of thousands of people and caused widespread destruction, the suburb of Waer is the last redoubt of the rebels.