Russia is backing with air support a Syrian government offensive to retake the major city of Aleppo from rebel fighters, according to Agence France-Presse. The support for Syrian President Bashar Assad follows weeks of Russian airstrikes in the country, including Wednesday bombings targeting the headquarters of Jabhat al-Nusra, an al Qaeda affiliate and one of the strongest rebel groups in Aleppo.

The Syrian government has launched four offensives in Aleppo since Russia joined the fight Sept. 30. Aleppo has been divided between government and rebel control since mid-2012, and was the economic hub of the country before the civil war broke out following the Arab Spring uprising in early 2011. The Islamic State group, also known as ISIS, also has a presence in Aleppo and has been making gains recently.

"A major military operation began on Friday in the southern parts of Aleppo province with the participation of our allies and our friends," a Syrian security source told Agence France-Presse, explaining that "allies" was meant to refer to Russian air support and "friends" were Iranian and Hezbollah fighters.

Russia's overt entrance into the Syrian conflict last month drew concern that the airstrikes and support for Assad's regime would lead to conflict with the United States and NATO allies like Turkey. Initial Russian strikes met criticism from the U.S. and NATO, which said lack of coordination with their own attacks were risky and could lead to midair disasters. 

Coordination to avoid incidents in Syria is still unresolved. Russia has criticized the U.S. for a failure to coordinate and shunning discussions beyond just avoiding midair clashes. The Obama administration has long said that Assad must go.