Syrian government forces swept through the Old City of Aleppo on Wednesday as rebel forces — besieged and facing certain defeat — debated when to withdraw from their shattered stronghold.

The government’s push into the historic heart of Syria’s largest city marks a defining moment in more than five years of war. The army and allied militiamen now control three-quarters of east Aleppo, the rebels’ most important enclave.

The accelerating rebel collapse came as Secretary of State John F. Kerry and Russian Foreign Minister Sergei Lavrov met in Germany for a last-ditch effort to agree on conditions that would allow the city to be evacuated. Kerry said the two would meet again Thursday morning.

More than 730 people have been killed in Aleppo since the government offensive began on Nov. 15, according to the Syrian Observatory for Human Rights, a monitoring group. On Wednesday, the militants called for a five-day cease-fire to allow civilians, including an estimated 500 people in need of medical evacuation, to leave for the countryside north of the city.

But officials within the armed opposition, speaking on the condition of anonymity, said a much broader agreement was being discussed involving a full withdrawal from the city. Activists said dozens of rebels had already fled.

In comments published Wednesday, President Bashar al-Assad said victory in Aleppo would be a “huge step” toward the end of Syria’s war.

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

Repeated government warnings in recent weeks — sent via text message or printed on airdropped leaflets — have urged residents to leave, warning that those who stay will be “annihilated.”

In the winding, close alleys of Aleppo’s 12th-century Old City, history has been erased. The 14th-century Old Souk has burned, and at the Umayyad Mosque, metal spikes twist out where a minaret once stood.

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. 

“Even by Syrian standards, the recent bombardment and shelling have been the most intense in Aleppo,” Hanaa Singer, UNICEF’s representative in Syria, said in an emailed statement Wednesday.

The Syrian war has left almost half a million people dead and spurred the greatest refugee crisis since World War II. It has also become a proxy battleground. 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, diplomacy has focused mostly on stanching the crisis in Aleppo. 

President Obama and the leaders of Germany, France, Britain, Canada and Italy on Wednesday signed a public statement saying they “condemn the actions of the Syrian regime and its backers, especially Russia,” for the Aleppo attacks and their refusal to allow humanitarian aid into the city and other besieged areas of Syria.

The Western leaders called on Syria and Russia to agree to a plan proposed by the United Nations for a cease-fire, humanitarian aid and talks on a transition government between the opposition and Assad. France will host a meeting in Paris on Saturday for opposition supporters from Europe and the Middle East; Kerry will attend.

The leaders’ statement came after Russia and China on Monday vetoed a U.N. Security Council resolution calling for a seven-day truce in Aleppo. It was the sixth time that Moscow has blocked U.N. action on Syria.

In Germany, no progress was reported. Lavrov emerged from his meeting with Kerry in Hamburg after just over an hour, saying he had to leave for another appointment. Kerry told reporters that the two had “obviously talked about the extraordinarily dire situation in Aleppo, and we exchanged some ideas about it and we intend to reconnect in the morning to see where we are.”

Asked before the meeting if Russia would support a cease-fire, Lavrov told reporters as the meeting began that he had agreed to support the “American proposal of the 2nd of December.” In a meeting between the two Friday, Kerry proposed the safe departure of civilians and thousands in the “moderate opposition,” leaving behind the al-Qaeda-linked group that Russia and Syria insist is the target of their attacks.

When the rebels balked at the deal, Russia said it assumed it was off the table, but Lavrov agreed to meet with Kerry in Hamburg, where both are attending a meeting of the Organization for Security and Cooperation in Europe.

The Kremlin confirmed Wednesday that a Russian military adviser in Aleppo died of wounds sustained in a rebel mortar attack. It was the third Russian fatality in Syria this week.

As pro-Assad forces moved in on the rebels’ final enclave, civilians there said they had nowhere left to run. In their voice messages, explosions crackled in the background. And when asked whether they were safe, residents repeated the same words: “I’m alive.”

DeYoung reported from Hamburg. Andrew Roth in Moscow and Zakaria Zakaria in Istanbul contributed to this report.