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.

Saturday’s fighting was centered around the southwestern neighborhood of Salaheddine, one of the first areas seized by the rebels since they began a push to control the city after being routed from the capital, Damascus.

Activists said helicopters were strafing the area and rebels faced artillery barrages and regime tanks trying to push into the neighborhood.

An Aleppo-based activist, Mohammed Saeed, said the government counterattack had begun and rebels were fighting back in several other areas.

“Thanks be to God, they haven’t succeeded in entering any of the (rebel-held) neighborhoods yet,” he said.

President Bashar Assad’s forces have been massing outside the city over the past few days, and Saeed said rebels from around the country also have been pouring in to help defend areas under their control.

“About 1,000 fighters have come from the Free Syrian Army from outside the province of Aleppo to help,” he said, referring to the main rebel group.

State television, in a rare comment on the situation in Aleppo, reported that government forces had inflicted heavy losses on groups of terrorists, the term the regime uses for the rebels.

The pro-government daily newspaper Al-Watan called it “the mother of all battles.”