Rebels defiant as Syrian army nears Aleppo's Old City

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.

Rebel groups in Aleppo have told the United States they will not leave their shrinking enclave, a senior rebel official told Reuters, after Russia call for talks with Washington over a full withdrawal of opposition fighters.

But the rebels 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 December 4's gains, some of which were confirmed by a rebel official with the Jabha Shamiya group, included a strategically important eye hospital. The rebel 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 rebel 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 U.N. Security Council is due to vote on December 5 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 spiraled 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 December 4 it was ready for talks with the United States over a full withdrawal of rebels from Aleppo, but speaking to Reuters from Turkey, senior rebel official Zakaria Malahifji of the Fastaqim group said rebels fighting in Aleppo had told U.S. officials on December 4 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 mobilized in Aleppo'," Malahifji said.

"They listened to the response and did not comment," he said, adding the rebel 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 rebel fighters "without exclusion" from Aleppo.