At least 15 people, including five children, were killed in a rocket attack on a market in the northern Syrian city of Al Bab on Friday, emergency workers said.
Thirty-eight others were wounded, the Syrian Observatory for Human Rights said in a statement.
Al Bab, located in the north-eastern countryside of Aleppo, is a rebel-held city.
The Syrian Observatory for Human Rights, an opposition war monitor, blamed Syrian government forces for the shelling, saying it was in retaliation for a Turkish air strike on Tuesday.
The Turkish air strike killed at least 11 Syrian troops and US-backed Kurdish fighters from the SDF.
“The number of martyrs is likely to increase due to the large amount of injuries, most of which are still receiving treatment and first aid in Al Bab city hospital,” said the observatory.
The shelling was carried out from Syrian government sites, the UK-based watchdog said.
The Observatory, which depends on a network of activists on the ground, reported that shells fell on residential areas and a popular market in the city.
The SDF, spearheaded by Kurdish groups who have opened a dialogue with the Damascus-based government, also control parts of the north and north-east.
The head of the SDF's media centre, Farhad Shami, said the group had nothing to do with Friday's attack.
The Turkish attack took place west of the northern town of Kobani and came amid tensions in northern Syria between US-backed Kurdish fighters and Turkish-backed opposition gunmen.
Fighting in Syria has decreased over the last few years, but shelling and air strikes are not uncommon, especially in the north as it is home to the last major rebel area in the country.
Since Syria’s conflict began in March 2011 hundreds of thousands have been killed and half of the country’s prewar population of 23 million has been displaced.
President Bashar Assad’s forces now control most of Syria, with the help of allies Russia and Iran.
