At least 23 people, including three children, were killed on Saturday in clashes near Damascus and airstrikes in northern Aleppo, a monitoring group said.
The Syrian Observatory for Human Rights said 13 people, including two children, were killed when Syrian government troops shelled the rebel-held town of Douma in the Eastern Ghouta region on the outskirts of Damascus.
The Britain-based watchdog, which relies on a network of activists inside Syria, said that fighting was raging between rebels and regime forces on the edge of Balaa district in Eastern Ghouta.
Local activists said that Eastern Ghouta is covered by the ceasefire brokered by the United States and Russia that started in parts of the country in February.
In the northern city of Aleppo, unidentified warplanes bombarded the rebel-held area of al-Bab, killing at least 10 people including a child, according to the Observatory, in the second consecutive day of strikes.
On Friday, at least 19 people were killed in airstrikes on the rebel-held neighbourhood of Bustan al-Qasr in Aleppo city.
Aleppo remains a key battleground in Syria's five-year-old conflict.
Control of the pre-war economic hub is divided between rebels in the east and the government in the west.
There is concern that the government plans to impose a siege on the rebel-held sections of the city, where tens of thousands of people live.
On Friday, UN envoy for Syria Staffan de Mistura called on international powers to help salvage the country's truce.
The ceasefire excludes the Islamic State extremist militia and the al-Qaeda-linked al-Nusra Front.
Earlier Saturday, Islamic State claimed to have captured a Syrian air force pilot after his plane crashed east of Damascus, a media outlet close to the radical group reported.
The pilot landed with his parachute and was near the plane when he was taken, the Amaaq news agency said.
Video footage posted online showed Islamic State fighters around the plane's wreckage. The video has not been independently verified.
There was no official comment in Damascus.
In January 2015, Islamic State released a video showing a Jordanian pilot being burned alive. He was captured when his plane crashed while carrying out airstrikes over the Islamic State stronghold of al-Raqqa, north-eastern Syria.