BEIRUT—Syria’s antigovernment rebels claimed a stunning advance into Aleppo, a major symbolic and strategic prize, in a setback for the government in Damascus and its allies Russia and Iran.Islamist-led insurgents poured into Aleppo two days after launching an unexpected strike from the country’s rebel-held northwest and eight years after the city fell to forces loyal to the government of Bashar al-Assad at the end of a long siege.

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.

The advance on Aleppo followed a shock offensive launched on Wednesday, as fighters swept through villages and towns.

Syrian rebels have reportedly breached the city of Aleppo itself, according to open source analysts and the Syrian Observatory for Human Rights, as thousands of insurgent fighters make startling advances in their surprise offensive against President Bashar al-Assad's regime in northwestern Syria.

It was the first time the city has been attacked by opposition forces since 2016, when they were ousted from Aleppo.