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 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," said 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 said.
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 United States 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 on Monday, the first day of the Muslim Eid al-Adha festival.
The agreement also paved the way for joint US-Russian raids against hardline groups in Syria, including the Islamic State of Iraq and the Levant (ISIL) and Fateh al-Sham, formerly known as al-Nusra Front.
READ MORE: Syria ceasefire deal explained
US Secretary of State John Kerry, emerging late on Friday in Geneva from talks with Russian Foreign Minister Sergey Lavrov, said the deal could provide a "turning point" in the conflict if the parties implemented it "in good faith."
The two powers back opposing sides in the conflict, with Moscow supporting the regime of President Bashar al-Assad and Washington backing a coalition of rebels it regards as moderate.
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.
Iran and the Lebanese armed group Hezbollah, both supporters of Assad, have supported the deal as well.
Iranian foreign ministry spokesman Bahram Ghasemi said on Sunday, however, that the success of the ceasefire relied on "comprehensive monitoring" of the truce, particularly along Syria's volatile borders.
The influential Ahrar al-Sham rebel group late on Sunday rejected the truce deal, just hours before it was set to begin.
A high-ranking member of the group, which works closely with former Al-Qaeda affiliate Fateh al-Sham Front, said in a statement on YouTube that the deal would only serve to "reinforce" the government of President Bashar al-Assad and "increase the suffering" of civilians.
The opposition's political umbrella group, the High Negotiations Committee (HNC), cautiously welcomed the agreement, but said it required "effective enforcement mechanisms" if any truce deal is to "credible".
"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."
Kodmani told Al Jazeera that if the truce was to be credible, any ceasefire violation should be met with a "military response".
"If we we have a credible line of cessation of hostilities, then we can look to moderate groups in the opposition to disassociate themselves from extremists."
In a letter sent to Syrian rebels over the weekend, US State Department envoy Michael Ratney urged armed opposition groups to distance themselves from Jabhat Fateh al-Sham, formerly known as al-Nusra Front, or there would be "severe consequences".
Reuters news agency reported on Sunday that Syrian rebel groups responded to Ratney's letter on Sunday, saying they would "cooperate positively" with a ceasefire, but voicing deep concerns over the details of the deal.
The agreement, the rebels wrote in a letter to the US, neglected besieged areas and was devoid of guarantees, monitoring mechanisms or sanctions for breaches.
The rebel letter also expressed concern at clauses in the agreement indicating that Syrian government jets would not be barred from flying until up to nine days after the ceasefire had taken effect.