Syria's army and allied militia advanced towards opposition-held areas of Aleppo's Old City on Sunday in an attack which a military source predicted would be over in a matter of weeks.
Western and regional states backing the rebellion against President Bashar al-Assad appear unwilling or unable to do anything to prevent a major defeat for those fighting to topple the Syrian leader, whose campaign to regain all Aleppo has been backed by the Russian air force and foreign Shi'ite militias.
Opposition groups in Aleppo have told the United States they will not leave their shrinking enclave, a senior opposition official told Reuters, after Russia call for talks with Washington over a full withdrawal of opposition fighters.
But the opposition may eventually have no choice but to negotiate a withdrawal from eastern Aleppo, where tens of thousands of civilians are thought to be sheltering, in the face of relentless bombardment and ground assaults.
The army said Sunday's gains, some of which were confirmed by a opposition official with the Jabha Shamiya group, included a strategically important eye hospital. The opposition official said it had yet to fall.
Loud explosions were heard in eastern Aleppo as night fell, Reuters journalists in the government-held part of the city said. The Jabha Shamiya official said further advances may force a opposition withdrawal to the southwestern corner of their enclave.
"The areas of Old Aleppo will be threatened to a great degree," the official said. "It is scorched earth."
Food and fuel supplies are critically low in eastern Aleppo, where hospitals have been repeatedly bombed out of operation.
The UN Security Council is due to vote on Monday on a draft resolution demanding an initial seven-day truce in Aleppo, which could then be renewed. But it was unclear if veto-power Moscow would block the resolution.
Restoring full control over Aleppo would mark the biggest triumph yet for Assad in a war that spiralled from protests against his rule in 2011. The campaign is one of the war's most ferocious, with hundreds reported killed in recent weeks alone.
Russia said on Saturday it was ready for talks with the United States over a full withdrawal of opposition from Aleppo, but speaking to Reuters from Turkey, senior opposition official Zakaria Malahifji of the Fastaqim group said opposition fighting in Aleppo had told US officials on Saturday they would not leave.
"Our response to the Americans was as follows: 'we cannot leave our city, our homes, to the mercenary militias that the regime has mobilised in Aleppo'," Malahifji said.
"They listened to the response and did not comment," he said, adding the opposition groups had reiterated calls for humanitarian corridors to be opened for the delivery of food and medicine and the evacuation of the wounded.
The United States has yet to comment on the proposal made by Russian Foreign Minister Sergei Lavrov for talks on the withdrawal of all opposition fighters "without exclusion" from Aleppo.
Army to regain Aleppo "in weeks"
The opposition said the Russians had retreated from proposals agreed at talks with opposition groups in Turkey that would have resulted in jihadist fighters leaving the city, a ceasefire and humanitarian aid deliveries.
The Syrian army has vowed to crush the opposition and take back Aleppo.
"The expectation is weeks…The Syrian Arab Army will continue to implement its missions until the elimination of the terrorists and the recovery of control over all the eastern districts," the military source said.
The UN estimates that close to 30,000 people have been displaced by the latest fighting, 18,000 to government-held areas, a further 8,500 to the Kurdish-controlled neighbourhood of Sheikh Maqsoud and the rest within opposition-held areas.
UN envoy Staffan de Mistura has said more than 100,000 people may still be in the opposition-held area. The Syrian Observatory for Human Rights, a Britain-based group that reports on the war, said it could be as many as 200,000 people.
The opposition, including foreign-backed groups, say they have been abandoned to their fate in a war against better armed enemies including the highly trained Lebanese group Hezbollah.
People inspect homes
In another blow to the opposition, the head of a new opposition alliance in Aleppo, "The Aleppo Army", was seriously wounded on Saturday, the opposition officials said.
The Syrian Observatory for Human Rights said the army or its allies were attacking in at least eight places in effort to deplete their ammunition and manpower.
Residents who fled eastern Aleppo for government-held areas early in the war began returning to the Hanano district recently captured from the opposition to inspect their homes.
The government took journalists to the district of northeastern Aleppo on Sunday, using a road through the city centre that was reopened two days ago.
A Reuters journalist said buses leaving from western Aleppo were bringing a steady stream of people to inspect homes they had not seen in years. Russian military trucks also delivered aid to the captured eastern districts of eastern Aleppo.