The Syrian army said on Saturday dozens of its soldiers had been killed in a major attack led by Islamist Hayat Tahrir al-Sham rebels who swept into the city of Aleppo, forcing the army to redeploy in the biggest challenge to President Bashar al-Assad in years.

Over a few days, rebel fighters in northern Syria have launched incursions into several major cities with little resistance from government troops.

The Hayat Tahrir al-Sham rebel group in Syria has taken control of much of Aleppo city after local forces withdrew to organize a counteroffensive to retake Aleppo.

Rebel forces in Syria have seized control of most of the city Aleppo after launching a surprise offensive against government forces, a war monitor reported on Saturday as Russia says it deployed fighter jets against rebels in Syria.

The lightning assault on Aleppo by the Islamist militant group Hayat Tahrir al-Sham abruptly redrew the front lines of Syria’s civil war.

The Syrian army faces a major challenge as rebel forces, led by Hayat Tahrir al-Sham, launch a surprise attack on Aleppo, shaking the frozen frontlines. The army redeploys to counter the offensive, aiming to restore control.

The Hayat Tahrir al-Sham rebel group in Syria has taken control of much of Aleppo city after local forces withdrew to organize a counteroffensive to retake Aleppo.

Video posted by pro-opposition media in Syria shows militants in Aleppo’s western suburbs removing the Syrian national flag from a monument.

Syria state media say government troops are battling insurgents inside the country’s largest city Aleppo for the first time since 2016