Syrian government forces swept through the Old City of Aleppo on Wednesday morning as rebel forces - besieged and facing certain defeat - debated withdrawal from their shattered stronghold.

The army and allied militiamen now hold three-quarters of east Aleppo, four years after the area fell from government control.

The Syrian Observatory for Human Rights, a monitoring group, said forces allied to Syrian President Bashar al-Assad took over the Old City after rebels withdrew into the remains of their territory.

As the United States and Russia prepare talks on the prospect of a full rebel withdrawal from Aleppo, the militants themselves are at loggerheads over what to do next.

On Wednesday, they called for civilians to be allowed to leave for the northern countryside during a five-day cease-fire.

But sources within the armed opposition, speaking on the condition of anonymity because of the fractious nature of the negotiations, said a much broader agreement was in the cards, involving full withdrawal from what was once their most important stronghold.

In 2012, the capture of east Aleppo was seen as one of the rebels' biggest victories. It turned out to be a fatal overstretch, making this final confrontation for the second city inevitable.

The eastern districts have been under siege since July, with Syrian and Russian warplanes carrying out endless rounds of bombardment, killing hundreds of civilians and destroying hospitals that treated the wounded.

More than 730 people have been killed in Aleppo since the start of a Nov. 15 government offensive and 80,000 have fled, the Syrian Observatory added Wednesday.

Repeated government warnings - sent via text message or printed on airdropped leaflets - have urged residents to leave, warning that those who stayed would be "annihilated."

The recapture of Aleppo would mark Assad's greatest victory in Syria's 5 1/2-year war. Even though morale in the east has been shredded by intense airstrikes and a crippling government siege, many civilians still say they fear for the future that awaits them with a government victory.

Inside what remains of the rebel enclave, there is a growing humanitarian crisis. Amid blistering bombardment, thousands of residents displaced by the offensive now shelter inside abandoned apartment blocks.

Food has almost run out, and fuel stocks are so low that rescue workers say they are often unable to reach the wounded. Photographs from the area Wednesday showed several bodies piled outside a hospital.

The Syrian war has killed almost half a million people and spurred the greatest refugee crisis since World War II. It has also become a proxy battleground for geopolitical rivals. While Iran and Russia have kept Assad afloat, the United States, Turkey and the Persian Gulf states have offered varying degrees of support to groups that took up arms against the government in 2011.

In recent months, that knotty diplomacy has focused mostly on stanching the crisis in Aleppo.

Secretary of State John F. Kerry was due to meet Russian Foreign Minister Sergei Lavrov in Hamburg on Wednesday to discuss an internationally brokered end to the fighting there.

But Moscow has also tried to sideline Washington, sending representatives for talks with rebel officials in Turkey. Col. Abo Bakr, a Free Syrian Army representative present at negotiations, said a provisional deal, including a cease-fire and the withdrawal of al-Qaeda linked militants, was agreed to before talks fell apart as government troops continued their sweep through the city.

"It became clear the Russians were procrastinating. They weren't really negotiating, they were buying time and waiting for the military situation on the ground to become irreversible," Bakr said.

The Kremlin confirmed Wednesday that a Russian military adviser in Aleppo had become the third Russian fatality in a week, dying from wounds sustained in a rebel mortar attack days earlier.

As pro-Assad forces moved ever closer to retaking the rebels' final stretch of territory, civilians there said they had nowhere left to run. Sending voice messages from the besieged enclave on Wednesday, Wissam Zarqa, an English teacher, spoke as warplanes echoed in the background.

He said: "I guess this means we can't leave now."