Speaking to Anadolu Agency, Najib Ansari, a civil defense official based in Aleppo, put the total number of casualties at 86.

According to sources at local field hospitals who spoke anonymously due to fears for their safety, warplanes struck residential neighborhoods in the city’s Al-Ansari, Al-Sokkari, Tariq al-Bab and Al-Shear districts -- all of which remain under siege by regime forces.

Ibrahim Abu Laith, another Aleppo-based civil defense official, told Anadolu Agency that Russian jets had targeted civil defense centers and fire departments in the city’s Al-Ansari and Masaken Hanano districts.

A number of ambulances, he added, had been destroyed by the airstrikes.

At the White House, spokesman Josh Earnest said if a nationwide cease-fire that calmed the situation in Syria for about a week has any chance of going back into force, "Russia's going to need to step up and prove it. And they've got some work to do."

Syria has been locked in a devastating civil war since 2011, when the Assad regime cracked down on pro-democracy protests -- which had erupted as part of the "Arab Spring" uprisings -- with unexpected ferocity. 

The Syrian Center for Policy Research, a Beirut-based NGO, has put the death toll from the six-year-old conflict at more than 470,000.