BEIRUT – A new rebel group boasting some 1,000 fighters launched an operation Sunday to capture Syria’s largest city, Aleppo, while government troops using helicopter gunships and heavy artillery rolled back opposition gains in the capital Damascus.

The spread of fighting into a second major metropolis displayed the rebels’ growing confidence even though they still can’t hold ground against the government’s heavy weapons, pushing Syria’s civil war toward a new phase of destructive urban combat.

On Sunday, a group calling itself the “Brigade of Unification” announced in an online video that it was launching an operation in Aleppo, Syria’s most populated city that has remained relatively quiet throughout the uprising.

“We gave the orders to march on Aleppo with the aim of liberating it,” says Col. Abdul-Jabbar Mohammed Akidi, one of the group’s leaders.

The push into Aleppo follows weeks of high-level military defections, soaring death tolls, fierce fighting near President Bashar Assad’s seat of power and a bomb blast that killed four top players in his regime’s efforts to crush those seeking to end his rule. Rebels also captured several border crossings with neighboring Iraq and Turkey. The opposition’s momentum put the regime on the defensive for the first time in the 16-month conflict.

But while the gradual swelling of the rebels’ ranks and increasing organization have allowed them to push into major cities, they remain largely unable to hold ground against Assad’s forces and helpless before his helicopters.

The escalating fighting is also feeding fears that Syria’s war could spill across borders and spark a regional conflagration. Assad’s regime is a bridge between Iran and the Lebanese Shiite militant group Hezbollah. On the other side, the uprising is largely driven by Syria’s Sunni Muslim majority, which has more natural links with the region’s Sunni nations such as Saudi Arabia.