Turkish President Recep Tayyip Erdogan has accused Russia of trying to set up a "boutique state" in Syria.
Speaking to journalists in Istanbul on February 5, Erdogan said Moscow "must be held accountable for the people it has killed within Syria's borders." He added that Moscow and Damascus are collectively responsible for 400,000 deaths in Syria.
According to UN figures, some 260,000 people have been killed since the civil war in Syria began.
Erdogan also said that a recent statement from Moscow that Turkey was planning to invade Syria was "laughable."
Meanwhile, the UN's Office for the Coordination of Humanitarian Affairs said the same day that some 15,000 refugees from northern Aleppo have massed on the Turkey-Syria border.
Turkish Prime Minister Ahmet Davutoglu said on February 4 that as many as 70,000 refugees from Aleppo are now headed toward Turkey.
Tens of thousands of people have fled the city after days of fierce fighting in which government forces cut off the main supply line to the rebels in Aleppo.
Russia launched a campaign of air strikes on September 30 that has helped Assad's forces to take control of several key rebel strongholds and has bolstered their assault on Aleppo, Syria's second-largest city.