At least 500 people have been killed on all sides during fighting in the Aleppo province since the start of an offensive by the Syrian regime and allied forces in early February, a war monitor said on Wednesday.
The Syrian Observatory for Human Rights said this figure included at least 100 civilians.
Tens of thousands of Syrians have been fleeing to the Turkish border north of Aleppo since last Thursday, following heavy Russian air strikes and reports of Syrian regime forces and its allies making gains in Aleppo's northern countryside, cutting opposition supply lines to Turkey.
Russia's intervention has tipped the balance of the war in favour of Syrian regime leader Bashar al Assad, reversing gains the opposition made last year. Advances by the Syrian regime forces and allied militant groups, including Iranian forces, are threatening to cut off opposition-held zones of Aleppo.
Taking full control of Aleppo would be a huge strategic gain for the Assad regime in the five-year conflict that has killed at least 260,000 people across the country and driven out more than half of the Syrian population.
It could also push a massive new wave of refugees to the Turkish border. Turkey has kept an open-door policy to civilians fleeing Syria throughout the conflict but is coming under growing pressure from Europe to stem the flow of refugees and from the United States to secure the border more tightly.
Turkish President Recep Tayyip Erdogan said on Thursday that "as required by our conscientious responsibility, we house over 2.7 million Syrian and Iraqi refugees. Turkey houses the most refugees in the world. Despite its heavy cost, we maintain our open door policy."
Erdogan also underlined that the flow of Syrian refugees into Europe will not stop as long as Russian and Syrian regime air strikes kill civilians in the country.