The Syrian government launched a major offensive from the north of Aleppo and captured several strategically important towns, as talks in Geneva on a ceasefire continued on Tuesday.
Monday's advance opened the way for President Bashar al-Assad's forces to retake Aleppo, Syria's largest city, as troops marched through the towns of Hardatnein, Tal Jibbeen, and Deir Zaitoun.
Hundreds of families were reportedly fleeing their homes because of heavy Russian air strikes on the area assisting the Syrian government's advance.
Opposition leaders in Switzerland condemned the offensive, saying that it shows the government is not committed to finding a peaceful resolution to the civil war.
John Kerry, the US secretary of state, also warned on Tuesday that "the situation on the ground for the Syrian people is unfathomable".
"We haven't seen a catastrophe like this since World War II, and it's unfolding before our eyes," Kerry said at a conference in Rome on countering the advances of the Islamic State of Iraq and the Levant (ISIL) group.
Talks in Geneva continued on Tuesday as the UN special envoy for Syria, Staffan de Mistura, was to meet representatives of Assad's government.
Meanwhile, Syrian opposition leaders were meeting a Russian deputy foreign minister.
Discussions are meant to develop a "road map" to end the nearly five-year conflict that has resulted in more than 250,000 Syrians being killed.
