The Britain-based Syrian Observatory for Human Rights says intense fighting is taking place Sunday in the city's southwestern neighborhoods.
The group said more than 25 people were killed Saturday in the commercial hub and Syria's largest city, among more than 160 killed in anti-government related unrest across the the country.
The U.N. has said more than 17,000 people have been killed in Syria since clashes broke out in March of last year.
U.N. special envoy Kofi Annan said Saturday he feared an "imminent battle" for Aleppo. Both France and Russia echoed his concern about the mounting violence.
French President Francois Hollande renewed a call for the U.N. to intercede, saying the role of U.N. Security Council member states is to intervene "as quickly as possible."
Earlier, Russia warned a "tragedy" was imminent in Aleppo, but said foreign support of rebels would lead to "more blood."
Some information for this report was provided by AP, AFP and Reuters.
