BEIRUT, Lebanon — Bags of body parts. Three young siblings killed along with their mother. A pregnant woman lying dead under a fallen roof.

These sights — described by Syrians after a marketplace bombing in the northern Syrian city of Idlib on Saturday — represent just a small fraction of the carnage from Russian or government airstrikes in the two days since Russia and the United States declared that a new cease-fire would begin on Monday.

At least 91 people were killed and scores more wounded in two days of attacks on rebel-held areas around the country, mainly in Idlib and in the divided city of Aleppo, according to tallies by doctors, rescuers and monitoring groups. The violence has deepened mistrust among Syrians that the cease-fire, agreed on late Friday, will deliver on its promise to ground the government planes that opposition groups say cause the war’s greatest death toll.

Rebels, who have no air power, also attacked government-held areas, mainly with mortar shells. The Syrian state news media reported rebel shelling in several cities — Damascus, Aleppo, Hama and Dara’a — but did not say whether there were casualties.