Residents told Al Jazeera that government forces summarily executed dozens of civilians over alleged connections to opposition fighters. The figure could not be independently verified by Al Jazeera.
A summary execution is an execution in which a person is accused of a crime and immediately killed without benefit of a full and fair trial.
The US state department said on Monday that Russia told the US it wanted a ceasefire in east Aleppo delayed for several days, a proposal the agency said was unacceptable in view of the continued attacks on civilians.
"Rather than accepting the US proposal for an immediate cessation, the Russians informed us that a cessation could not start for several days, meaning that the assault by the regime and its supporters will continue until any agreement will go into effect," John Kirby, state department spokesman, said in response to a question about weekend talks in Geneva.
"Given the dire situation in Aleppo and the reports of continued attacks on civilians and infrastructure, this was just simply not acceptable."
Kirby said US and Russian officials were continuing their talks in the Swiss city, although there was for now an unbridgeable gap between their positions.
"Our teams are still trying to work this out," he said. "While we would still like to get there ... I think we're certainly at an impasse right now."