Forces backing Syrian President Bashar Assad pressed their offensive Tuesday on Aleppo's rebel-held zone from the south, after capturing areas on other fronts in recent days. As reinforcements arrived, including Shiite fighters from Iraq, the strategy appeared to be to retake rebel-held areas bit by bit, backed by massive Russian airpower, rather than risk a potentially costly all-out ground battle.

Tuesday's offensive on the city's besieged rebel-held eastern neighborhoods came a day after Washington suspended direct U.S.-Russian talks on a Syria cease-fire — a move U.S. Secretary of State John Kerry blamed on Russia's rejection of diplomacy in favor of helping Assad's government achieve a military victory over the rebels.

The latest tactic of whittling away at rebel-held areas of Aleppo rather than launching an all-out offensive has proved successful in the past: The government reasserted control of the suburbs of the capital, Damascus, and most of the central city of Homs using the strategy.

"The Syrian army and its allies are in a sustained offensive to recapture rebel-held eastern Aleppo," wrote Robert Ford, a veteran diplomat and former ambassador to Syria.

"Unless the balance on the ground drastically shifts, the Assad regime will eventually retake from opposition fighters all of Aleppo and the outlying districts of Damascus," wrote Ford, a fellow at the Middle East Institute in Washington. "This may take months, but the balance is certainly in the Syrian government's favor."

"Aleppo is ... the Syrian crisis and its liberation will end plans to divide Syria," agreed Amin Hoteit, a former Lebanese army general and expert on military and strategic affairs.

Syrian troops and their allies have laid siege to rebel-held parts of Aleppo since July 17, except for a few weeks when the militants were able to break it in August, until it was re-imposed in early September. Soon after the government opened a corridor for civilians and fighters to move to government-held parts of the city, and dozens of people and gunmen crossed after a general amnesty was offered by authorities.

Since a cease-fire brokered by the U.S. and Russia ended on Sept. 19, rebel-held neighborhoods where 275,000 people live have been subjected to some of the worst bombardment by Russian and Syrian warplanes since fighting began in 2011. Hospitals have been among the hardest-hit targets.

At least 420 people have been killed and more than 1,000 wounded in and around Aleppo since the cease-fire collapsed, according to the Britain-based Syrian Observatory for Human Rights. Most of the deaths were in eastern Aleppo, where scores of buildings were demolished by Russian and Syrian airstrikes.

"The regime is bombing civilians because of its inability to storm Aleppo for years," said opposition activist Abu Firas al-Halaby, adding that talk of the imminent arrival of reinforcements was part of a "psychological war" against the rebels.

The Syrian government is backed in the Aleppo battle by Lebanon's Hezbollah, the Palestinian Quds Brigade and Iraq's Shiite al-Nujaba militia, among others.

An official with the al-Nujaba militia said the group had sent some 4,000 fighters in recent weeks in preparation for a battle to storm Aleppo's eastern neighborhoods. The official, who spoke on condition of anonymity because he was not authorized to discuss military strategy, refused to say when such an attack might take place.

Captain Abdel-Salam Abdel-Razek of the Nour el-Din el-Zinki rebel group said a wide ground offensive was unlikely because the government can't match the rebels' advanced street fighting.

"I don't expect that. I expect attrition and attempts on more than one front. But I don't see that it has the capacity to carry out a wide offensive on all fronts," Abdel-Razek said. "As a force on the ground, it is mobilizing and it can possibly later, but not now."

Government forces have made some progress on the ground recently, including last week's capture of the Handarat Palestinian refugee camp north of Aleppo and the nearby Kindi Hospital, which overlooks a key intersection of vital roads. Syrian troops also captured the central neighborhood of Farafra after pushing forward from the Old City.

On Tuesday, government forces and their allies attacked the southern rebel-held neighborhood of Sheik Saeed, opening a third front against the rebels.

An Aleppo-based activist, Baraa al-Halaby, said residents now avoid underground shelters, for fear of bunker-busting bombs that penetrate several floors before exploding. Still, he said government forces will not be able to storm rebel-held neighborhoods of the city.

"Tanks will not be able to advance easily in the streets," said al-Halaby, adding that the Handarat camp was only taken because it is an open area.

Demonstrating Moscow's commitment to backing Assad, the Russian military said it has beefed up its forces in Syria with state-of-the-art air defense missiles.

The Defense Ministry said Tuesday that a battery of the S-300 air defense missile systems has been deployed to Syria to protect a Russian navy facility in the Syrian port of Tartus and Russian navy ships in the area.

The deployment will add punch to the Russian military force in Syria, which has long-range S-400 missile defense systems and an array of other surface-to-air missiles at the Hemeimeem air base in Syria's coastal province of Latakia.

Still, Jennifer Cafarella of the Study of War, a Washington-based institute, said the fight for Aleppo will not end quickly.

"It has taken much of the regime combat power and much of the combat power that Russia has brought to theater in the form airstrikes, and the Iranian ground reinforcement," she said.

"Those military forces are going to be occupied in Aleppo for some time. But after that I do expect that Damascus can become the focus."