Russia has accused the United States of canceling a meeting this week to discuss the fate of dwindling rebel forces in Syria's Aleppo, and declared that fighters who refuse to leave the city will be "eradicated."
The United States rejected the accusation, saying it had never confirmed talks would take place this week.
The charge came as Russian-backed Syrian forces made rapid headway in retaking rebel-held areas of the city.
Russian Foreign Minister Sergei Lavrov said U.S. negotiators had "changed their mind" and canceled a meeting he set for December 6 or 7 in Geneva to discuss an orderly withdrawal of rebel forces from the city.
Lavrov said the United States sent Russia a new proposal that "takes everything back to square one and looks like an attempt to gain time to let militants take a break and replenish their reserves."
In light of what he cast as U.S. backtracking, he said that Russian and Syrian forces will "eradicate" any opposition fighters left in the city.
U.S. State Department spokesman Mark Toner countered that "talks haven't been canceled. We never were in a position to confirm that these talks were taking place this week."
Toner added that "we're still pursuing those, we're just not in a position yet where it [makes] sense for us to meet."
U.S. Secretary of State John Kerry conceded that rebels might lose control of Aleppo, but he said negotiations would still be needed to end of the war.