The Syrian army has seized two-thirds of east Aleppo and continued to advance on Monday, pounding remaining opposition-held territory.
Russian Foreign Minister Sergei Lavrov said earlier that Moscow would hold talks with Washington this week on a timeframe for the withdrawal of all rebels from Aleppo.
"During the Russian-American consultations, the concrete route and timeframe for the withdrawal of all fighters from eastern Aleppo will be agreed upon," he said, indicating the talks in Geneva would likely start on Tuesday or Wednesday.
"As soon as these routes and timeframes are agreed on, a ceasefire can come into effect," Lavrov said.
But officials from two rebel groups in Aleppo said they would reject any plan that involved the withdrawal of fighters from the city.
Yasser al-Youssef of the Nureddine al-Zinki rebel group said any proposals "for the exit of rebel groups would be unacceptable."
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"It is for the Russians to leave Aleppo, and for the sectarian militias to leave Aleppo and Syria and stop interfering in the internal affairs of Syrians," he told AFP.
Russia is a staunch ally of Syria's government, and it launched a military campaign to bolster President Bashar al-Assad's forces in September 2015.
The government also fights alongside Lebanon's Shiite Hezbollah movement, Iranian forces and Shiite fighters from other countries.
"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.
"This is our land and the land of our ancestors, and we will stay on it, and defend it, God willing," he said. "The revolution will continue until victory."
Both officials said rebels remained willing to approve a UN plan for the entry of humanitarian aid into the east, which has been besieged by government forces since mid-July.
Rebel forces seized east Aleppo in 2012, and the army last month launched a major operation to recapture it.
So far it has seized two-thirds of the east in a ferocious assault that has killed nearly 320 civilians and over 200 rebels, according to a monitoring group.