BEIRUT – Syrian government tanks backed by attack helicopters swept into Aleppo on Saturday as the regime launched an assault to regain control of the country's largest city a week after rebels seized several neighborhoods.

The high-stakes battle for Aleppo, a commercial hub and the country's largest city, has raised fears among activists and the international community that a new massacre could be looming.

Even Syria's longtime ally, Russia, added to the chorus of alarm Saturday, saying a "tragedy" was imminent in Aleppo. But Russia's foreign minister said it was unrealistic to expect the Syrian army to stand by while rebels were trying to take over major cities.