Bassem Mroue ?and Sarah El Deeb

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 indiscriminate 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 their allies capture the rebel-held eastern neighborhoods, it would be a turning point in the 5½-year-old civil war that has killed more than 250,000 people and displaced half of Syria’s population.

Over the course of the conflict, the government has slowly regained control of major cities. Its aim appears to be securing what some analysts call “useful Syria” — a portion containing the four largest cities of Aleppo, Damascus, Homs and Hama, along with its Mediterranean coast.

Aleppo is the last of the major cities still being contested, and it could take government forces between six months and a year to capture it, unless they aim to “annihilate” the politically significant city, a Western diplomat told the Associated Press. The envoy, who is familiar with the cease-fire talks that have faltered, spoke on condition of anonymity because of his government’s regulations.

Once all of “useful Syria” is in government hands, international diplomacy would have to determine the fate of the jihadi-controlled northwest and those areas dominated by the main Kurdish militia and the Islamic State militant group.

President Bashar Assad “doesn’t want a negotiation,” the diplomat said, adding that “the Russians wouldn’t or couldn’t stop him” from attacking Aleppo.

In Russia, Deputy Foreign Minister Mikhail Bogdanov told Syria’s ambassador that Moscow has “a firm intention to continue providing assistance to the Syrian government in fighting terrorism and to help achieve the soonest possible political settlement of the Syrian crisis.”

Opposition forces control almost half of Aleppo, the only major city where rebels hold such a large area. The U.N. says more than 250,000 people live in the rebel-held areas, while more than 1 million are in the government-controlled part that is usually subjected to shelling.

Assad’s government controls the capital of Damascus, except for two small neighborhoods. It also controls all of Homs and Hama, the third- and fourth-largest cities.