The Syrian army and its allies made considerable advances in Aleppo on Monday while constantly bombarding rebel areas in pockets packed with civilians.
Syrian President Bashar al-Assad is now close to taking back full control of Aleppo after nearly six years of conflict as pro-government forces clashed with insurgents in the Fardous district
"The situation is extremely difficult today," Zakaria Malahifji of the Fastaqim rebel group fighting in Aleppo was quoted as saying by Reuters.
The rebels' sudden retreat represented a "big collapse in terrorist morale", a Syrian military source said.
Though a US-Russian proposal for the withdrawal of fighters and civilians from the city's opposition areas was tabled for rebel groups in Aleppo on Sunday, Moscow denied that any agreement had been reached.
Rebel officials blamed Russia, saying it had no incentive to compromise while its ally Assad was gaining ground. "The Russians are being evasive. They are looking at the military situation. Now they are advancing," he said.
Meanwhile tens of thousands of civilians remain in rebel-held areas without basic supplies and are at risk of being pounded by air strikes or caught in the shelling, according to the Syrian Observatory for Human Rights, a British-based monitoring group.
In the past four weeks into the army offensive,the Observatory tallied at least 415 civilians deaths, including 47 children. Hundreds were injured by Russian and Syrian air strikes and shelling by government forces and its allies on the besieged eastern part of the city.
The Observatory said 364 rebel fighters had been killed in the eastern sector. It said rebel shelling of government-held west Aleppo had killed 130 civilians, including 40 children. Dozens had been injured.
Even with Aleppo's fall, Assad would still be far from restoring control across Syria as swathes of the country remain in rebel hands. On Sunday, ISIS retook Palmyra.
