Forces backing Syrian President Bashar Assad pressed their offensive on Tuesday on Aleppo's rebel-held zone from the south, after capturing areas on other fronts in recent days, a report said.

With new reinforcements, the strategy appeared to be to retake rebel-held areas, backed by massive Russian airpower, added the Fox News report.

The move came a day after the US suspended Syria ceasefire  talks with Moscow, which US Secretary of State John Kerry blamed on Russia's rejection of diplomacy in favour of helping Assad's government achieve a military victory over the rebels.

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

"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. "This may take months, but the balance is certainly in the Syrian government's favour."