The largest offensive in years by Syrian opposition fighters against government forces has stirred fears of reigniting a civil war that has been mostly frozen for years.

The new rebel push began Wednesday in Aleppo Province in northwestern Syria. On Thursday, the opposition forces advanced, capturing several new villages, according to a British-based monitoring group, the Syrian Observatory for Human Rights.

The offensive aims to stop attacks by government forces and their Iran-backed militia allies, a rebel commander said.

Two days of fierce clashes have killed more than 150 combatants from both sides: nearly 100 from rebel groups which launched the offensive and 54 regime soldiers, according to the Observatory. The group gathers information from a network of activists and others across Syria, and its numbers could not be independently verified.

In addition to those deaths, more than a dozen civilians have been killed by Syrian and Russian airstrikes, according to the White Helmets, a rescue group based in opposition areas. Russia and Iran have for years robustly helped President Bashar al-Assad’s autocratic regime stave off the rebels.