After losing two-thirds of the city, rebel forces have defied calls for a retreat. Russia says any forces left in the city will be dealt with.
Russia will hold talks with the United States this week to organize a complete rebel withdrawal from Syria's Aleppo, Foreign Minister Sergey Lavrov said Monday.
Forces loyal to Russian-backed Syrian President Bashar al-Assad have seized two-thirds of Aleppo since mid-November. The assault left opposition forces reeling, but they refused to back down.
Lavrov said he would begin talks in Geneva on Tuesday evening or Wednesday morning on a time frame and a route for evacuation. A ceasefire would follow successful talks.
"Those armed groups who refuse to leave eastern Aleppo will be considered to be terrorists," Lavrov told a press conference.
"We will treat them as such, as terrorists, as extremists, and will support a Syrian army operation against those criminal squads," he said.
But rebel forces rejected talk of an evacuation.
Yasser al-Youssef of the Nureddine al-Zinki faction, a leading rebel group in Aleppo, described the proposal as "unacceptable."
"The revolutionaries will not leave Aleppo and will fight the Russian and Iranian occupation until the last drop of blood," said Abu Abdel Rahman al-Hamawi of the Army of Islam, another rebel group active in Aleppo.
"No person in his right mind who has any sense of responsibility and patriotism would leave his city" Zakaria Malahifji from the Fastaqim rebel group said Monday.
Biggest loss yet
There was no immediate comment from Washington, which has backed some of the rebels and condemned the Syrian regime as perpetrating crimes against humanity.
If the US were to agree with Russia, it would be the first time that the powers have negotiated on the withdrawal of opposition forces in the war.
Rebel groups have reached deals with the regime when evacuating several other Syrian strongholds, most notably the 2014 exit from the Old City of Homs after a two-year government siege.
The latest attacks centered around the Old City and were aimed at cutting off rebel-held eastern Aleppo, where tens of thousands of civilians remained trapped.
About 15,000 rebel fighters were in the city before the assault began, according to the Britain-based Syrian Observatory for Human Rights. UN Syria envoy Staffan de Mistura put the number at 8,000 in October.
Government bombardments killed more than 300 people since mid-November, the Syrian Observatory reported. Rebel shelling killed 70 others.
aw/mkg (dpa, Reuters, AFP)