Speaking to Anadolu Agency, Najib al-Ansari, a pro-opposition civil defense official, said Russian fighter jets struck al-Marja district by “bunker-buster” bombs. 

Scores of people were injured in the attack, he said, adding that civil defense teams were struggling to dig up victims stuck under the debris of collapsed buildings. 

Russia has been using deep-penetration munitions ("bunker-busters") in Aleppo’s opposition-held neighborhoods. 

Described by locals as having the effect of “earthquakes”, such bombs are generally used to destroy heavily fortified underground shelters and tunnels. 

On Sunday, at least 49 people were killed and more than 100 others wounded when Russian and regime warplanes struck opposition-held areas in Aleppo. 

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

Since then, more than a quarter of a million people have been killed and more than 10 million displaced across the war-torn country, according to UN figures.

The Syrian Center for Policy Research, however, a Beirut-based NGO, puts the death toll from the five-year conflict at more than 470,000. 

One year ago, Russia, a main ally of the Assad regime, began extensive air operations in Syria with the stated aim of fighting terrorist groups based in the country and supporting the embattled Assad regime.