Monitoring group says air attacks hit rebel-held towns just 24 hours after a ceasefire was agreed.
Dozens of civilians have been killed in air strikes and artillery shelling in war-ravaged Syria, activists said on Sunday.
Air raids targeting Eastern Ghouta district outside the capital Damascus killed at least 23 people as Syrian government forces continue to increase military pressure on the rebel-controlled area.
“The toll could rise further because of the number of wounded people in a serious condition,” said Rami Abdel Rahman, director of the UK-based Syrian Observatory for Human Rights. At least four children were among the dead, he added.
The local Ghouta Media Center also reported 23 civilians had been killed.
Attacks on Eastern Ghouta over the past two weeks have killed more than 100 people, the Syrian Observatory said.
Sunday’s attacks targeted the opposition-held residential neighbourhoods of Douma, Misraba, Ein Tarma, Medeira and Harasta, according to an Anadolu news agency reporter in the area.
Rebels in Eastern Ghouta have managed to keep Syrian military forces at bay during years of war, however, a government siege of the districthas led to a humanitarian crisis with severe shortages of food and medicine.
Guney Yildiz, from the European Council of Foreign Relations, told Al Jazeera that Syrian forces are stepping up attacks against opposition fighters in advance of new negotiations in Geneva starting on Tuesday.
“I think the main goal is make rebels feel even weaker ahead of the UN talks next week,” Guney said.
He noted Eastern Ghouta, with a population of about 300,000, was a significant target because of its location so close to Damascus.
The Syrian Observatory also reported that Russian air strikes on Sunday killed 34 civilians, including 15 children, in a village held by Islamic State of Iraq and the Levant (ISIL, also known as ISIS) in eastern Deir Az Zor province.
The Russian raids reportedly hit al-Shafah village on the eastern bank of the Euphrates River early on Sunday.
Russia’s Defence Ministry said six Tu-22M3 long-range bombers targeted ISIL positions in Deir Az Zor province and struck “terrorist strongholds” in the valley near the river.
Deir Az Zor provnice is one of the final redoubts of ISIL in Syria after the armed group was pushed out of its main strongholds such as Raqqa and Deir Az Zor city.
Hundreds of thousands of Syrians have been killed in the war since it began in March 2011 with millions of residents fleeing the country.
Russia has been a major supporter of Syrian President Bashar al-Assad’s forces since its military entered the conflict in September 2015. The United States, Turkey, and Gulf nations have backed opposition fighters.