Rebel-held Eastern Ghouta near Damascus targeted with 23 people killed, while 34 die in Deir Az Zor, activists report.
At least 80 people have been killed in just over 24 hours in suspected Russian and Syrian government shelling in the eastern and southern parts of Syria.
The majority of those killed were civilians.
In the eastern province of Deir Az Zor, suspected Russian warplanes hit al-Shafah village on the eastern bank of the Euphrates River on Sunday, killing about 51 people.
Four others were killed in an attack on the village of al-Darnaj, also on Sunday, raising the death toll in the province to 55.
Deir Az Zor province is one of the final Syrian redoubts of the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) after the armed group was pushed out of its main strongholds, including Raqqa and Deir Az Zor city.
Meanwhile, in Eastern Ghouta, a district just outside the capital Damascus, at least 25 people were killed in alleged Russian and Syrian government shelling on several opposition-held areas on Sunday.
The attacks targeted a local market in the residential neighbourhood of Misraba, where 16 people were killed.
At least nine others were killed in shelling on the neighbourhoods of Medyara and Douma.
According to the UK-based Syrian Observatory for Human Rights (SOHR), a war monitoring group, the figure of 25 includes five children and three women.
Eastern Ghouta has consistently been targeted despite being listed as a “de-escalation zone”, where military activity is prohibited under an agreement endorsed by Turkey, Russia and Iran, which was signed in September.
But due to its strategic location near the capital, where the government of President Bashar al-Assad sits, the attacks there have not halted, in violation of the agreement.
Fadel Abdul Ghany, head of the Syrian Network for Human Rights, said Eastern Ghouta was the main target for the Syrian government now.
“The regime is not interested in de-escalating. After they took back Aleppo, they went to Deir Az Zor, and now it’s Ghouta. It’s their main target now,” Abdul Ghany told Al Jazeera.
Since the implementation of the agreement, at least 250 people have been killed in suspected Russian and Syrian government air raids in the district, according to medical centres and civil defence workers.
The armed opposition in Eastern Ghouta has managed to keep Syrian government forces at bay during the war. In turn, the government has imposed a siege on the area to pressure the armed opposition, causing a humanitarian crisis, with severe shortages of food and medicine.
Under the so-called “de-escalation zones” agreement, opposition groups and government forces are required to halt hostilities, including air raids, for a period of six months, to protect civilians living in those areas.
More than 2.5 million people are believed to be living in the zones, which include, fully or partially, Eastern Ghouta and the provinces of Idlib, Homs, Latakia, Aleppo and Hama.
Syria’s conflict evolved from a violent government crackdown on protests against the president in 2011 to become a devastating war that has drawn in world powers, including Russia and a US-led international coalition.
Russia has led negotiations on finding a solution to the conflict since its game-changing military intervention on the side of al-Assad in 2015.