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 surprise attack, led by the Islamist group Hayat Tahrir al-Sham, was the boldest rebel assault for years in the civil war that has been frozen.

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.

A rebel commander said a fall in Iran-backed manpower created an opening this week's assault. Iran's ally Hezbollah has suffered big losses to Israel.

Time and time again the Assad dynasty has survived internal and external challenges and lived to fight another day. But nothing lasts forever.