Air strikes on rebel-held areas of Syria reportedly killed at least 100 people hours after the US and Russia announced plans for a truce.
An air strike on a market in Idlib killed up to 60 people while at least 45 died in strikes on Aleppo province, opposition activists say.
A 10-day truce is meant to start on Monday, followed by co-ordinated air strikes against jihadist militants.
Turkey and the EU welcomed the plan but warned that further action was needed.
Turkey said aid must be delivered from the very start while EU foreign policy chief Federica Mogherini looked towards a “political transition”.
A spokeswoman for Syria’s opposition said the plan provided some hope but more details were needed about how it would be enforced.
In the capital, Damascus, the government endorsed the deal, the state news agency Sana reported.
There has been no official reaction from Iran which, like Russia, is allied to Syrian President Bashar al-Assad.
The conflict in Syria, which began with an uprising against Mr Assad, has raged for five years and claimed the lives of more than a quarter of a million people.
The truce is due to take hold at sunset on Monday, at the start of the Muslim holiday of Eid al-Adha.
Under the plan, Syrian government forces will end combat missions in specified opposition-held areas.
In an unexpected development, Russia and the US will then establish a joint centre to combat jihadist groups.
These include IS but also Jabhat Fateh al-Sham, the group known until recently as the Nusra Front, when it was allied to al-Qaeda.
The deal was reached in Geneva, Switzerland, between US Secretary of State John Kerry and his Russian counterpart, Sergei Lavrov.