The Syrian Observatory for Human Rights said strikes were conducted in the Aleppo region, the southern countryside of Hama, and near Raqqa, the de facto capital of ISIS.
It was unclear who conducted the airstrikes.
Russian state-run media, meanwhile, reported that six attacks on Damascus originated from areas held by the Syrian opposition fighters.
The United States and Russia helped lead the negotiation of a "cessation of hostilities" between a handful of rebel groups and the Syrian regime, which took effect Friday. Terrorist groups like ISIS and al Nusra Front are not part of the agreement.
A main Syrian opposition group, the High Negotiations Committee, said 97 of its factions agreed to respect the two-week truce that starts with the cessation of hostilities. But it warned the Syrian government and Russia not to target it under the pretense of attacking terrorist groups.
Salem Meslet, the spokesman for the High Negotiations Committee, told CNN's Nic Robertson that the Syrian regime has committed 15 violations of the cessation of hostilities agreement.
He blamed the airstrikes in Aleppo on Russian warplanes. The areas that were targeted do not have ISIS or al Nusra presence, he said.
The attacks by the Syrian regime have included barrel bombs and rockets, Meslet said.
"We worry (Syrian President Bashar al-Assad) will increase the violations if no one says anything to him," Meslet said. The High Negotiations Committee is sending letters detailing the alleged violations to the United Nations and all the members of the International Syria Support Group -- except for Russia and Iran.
Right now, Meslet said, "in general it is a lot better than before (the cessation of hostilities) and our people are more comfortable."
Russian state media said
that country halted airstrikes in certain parts of Syria in accordance with the cessation of hostilities agreement.
Russia's Interfax news agency said the alleged violations by the opposition in Damascus killed two people and wounded eight others.