Rebel forces in Syria have taken control of the “majority” of Aleppo, the country’s second-largest city, marking a significant escalation in the ongoing conflict, according to the UK-based monitoring group Syrian Observatory for Human Rights (SOHR).

For the first time since 2016, Russia launched airstrikes in parts of Aleppo overnight into Saturday, the observers reported. The SOHR estimates that more than 300 people, including over 20 civilians, have been killed since the offensive began on Wednesday.

This offensive represents the most substantial challenge to the Syrian government in years and marks the first time rebel forces have entered Aleppo since their expulsion by Syrian army forces in 2016.

Syria’s military confirmed on Saturday that rebels had seized control of “large parts” of the city. Dozens of soldiers have been killed or injured in the ongoing fighting. In response, the military announced a temporary withdrawal from Aleppo to “prepare a counteroffensive,” according to an official statement.

Aleppo’s airport and all access roads to the city have been closed, military sources told Reuters. The rebels reportedly captured much of the city with minimal resistance.

The rapid takeover of Aleppo has intensified concerns about the safety of civilians and the broader implications for the Syrian conflict, which has already claimed hundreds of thousands of lives. With Russian airstrikes and Syrian military maneuvers escalating, the battle for Aleppo signals a critical turning point in the war.

The situation remains fluid as the Syrian government prepares its counteroffensive and international observers monitor the fallout of the latest surge in violence.