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 11 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.

If government forces and their allies capture the rebel-held eastern neighborhoods, it would be a turning point in the 51/2-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 six months to a year to capture it, unless they aim to “annihilate” the city, a Western diplomat said.

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 over 250,000 people live in the rebel areas, while more than 1 million are in the government-controlled part.

Since the one-week cease-fire brokered by Russia and the U.S. ended Sept. 19, Aleppo has been under intense Russian and Syrian airstrikes, killing more than 200 civilians, knocking down entire buildings, disrupting water supplies and targeting Civil Defense centers.

Attempts to revive the cease-fire during the U.N. General Assembly failed, and U.S. Ambassador Samantha Powers criticized Russia, saying Moscow was practicing “barbarism.”