BEIRUT — With international diplomacy in tatters and the U.S. focused on its election, the Syrian government and its Russian allies are seizing the moment to wage an all-out campaign to recapture Aleppo, unleashing the most destructive bombing of the past five years and pushing into the center of the Old City.

Desperate residents describe horrific scenes in Syria’s largest city and onetime commercial center, with hospitals and underground shelters hit by airstrikes that the U.N. said may amount to a war crime.

Debris covers streets lined with bombed-out buildings, trapping people in their neighborhoods and hindering rescue workers. On Tuesday, activists reported at least 23 people killed in airstrikes on two districts in the rebel-held part of Aleppo.

The battle for Aleppo is unlikely to be an easy one for government forces because the isolated rebels say they are determined to “fight until the end” to defend their neighborhoods. Insurgents outside the city could also attack government troops to try to reduce pressure on comrades trapped inside.

If government forces and allies capture the rebel-held eastern neighborhoods, it would be a turning point in the 5½-year-old civil war.