Bombing raids on rebel-held parts of Aleppo and Idlib come after US and Russia agree ceasefire deal.
Ruinous violence has raged in several parts of Syria, shortly after the US and Russia sealed an ambitious agreement aimed at breathing life back into a stuttering UN-sponsored peace process.
More than 100 people were reported killed in a series of bombing raids on rebel-held parts of Aleppo province in the north of the country, and in Idlib in the north-west.
The worst strikes were in Idlib city, the capital of the province of the same name, where they hit a market, killing 55 civilians.
"A Russian fighter jet targeted a residential area and a market in Idlib," Al Jazeera’s Adham Abu al-Husam, reporting from the city, as civil defence forces, firefighters and paramedics worked to pull survivors from the rubble.
"The marketplace was full of civilians shopping for the upcoming Eid holiday."
In Aleppo, at least 46 civilians, including nine children, were killed in a bombardment of opposition-held areas, an Al Jazeera correspondent in the city reported.
The raids on Idlib and Aleppo were believed to have been carried out by Syrian army fighter jets, or those of its main ally Russia.
Aleppo, a major battleground in the conflict, has seen intensified fighting between government forces and the opposition in recent months, worsening the humanitarian situation there.
The surge in violence came hours after the US and Russia’s top diplomats announced the ceasefire agreement after 13 hours of talks in the Swiss city of Geneva.
The accord included a truce to start across Syria at sunset Monday, the first day of the Muslim Eid al-Adha festival. The agreement also paved the way for joint US-Russian raids against armed groups in Syria, including the Islamic State of Iraq and the Levant group.
US Secretary of State John Kerry, emerging late on Friday in Geneva from talks with Russian Foreign Minister Sergei Lavrov, said the deal could provide a "turning point" in the conflict if the parties implemented it "in good faith."
Russia is a major ally of Syrian President Bashar al-Assad, while the US is supporting opposition forces fighting to oust him.
Syria’s state news agency, SANA, said that the Geneva agreement had been reached "with full knowledge" of the Syrian government, which has approved it.
The Syrian opposition reacted with caution to the pact. The High Negotiations Committee (HNC), an opposition umbrella coalition, welcomed the agreement but called on Russia to pressure Assad’s government to comply with the deal.
"We hope this will be the beginning of the end of the civilians’ ordeal," HNC member Bassma Kodmani said.
"We welcome the deal if it is going to be enforced. What if Russia doesn’t pressure the regime, because that is the only way to get the regime to comply? We are waiting with a lot of anxiety."
A previous ceasefire, brokered by the US and Russia in February, collapsed.