Russia said Monday it would hold talks with Washington on a total rebel withdrawal from Syria's Aleppo, where the army has made sweeping advances, but opposition factions rejected any evacuation.
President Bashar Assad's forces have seized two-thirds of the former rebel bastion in east Aleppo since they began an operation to recapture all of the battered second city in mid-November.
The rapid gains for regime forces have left opposition fighters reeling and on Monday Russian Foreign Minister Sergei Lavrov said talks would be held on a rebel evacuation.
"During the Russian-American consultations the concrete route and timeframe for the withdrawal of all fighters from eastern Aleppo will be agreed upon," Lavrov said, adding that the discussions 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 rebel groups swiftly rejected any talk of an evacuation.
Yasser al-Youssef of the Nureddine al-Zinki faction, a leading rebel group in Aleppo, described any such proposal as "unacceptable."
"It is for the Russians to leave," he told AFP.
Moscow is a close ally of Assad's government, and launched a military intervention in support of Damascus last year.
Government troops have also been bolstered by Iranian forces, fighters from Lebanon's Hizbullah and Shiite fighters from other countries.