Rebels put up fierce resistance in the southeastern outskirts of the battered city, but government forces closed in on opposition territory from the east.
President Bashar al-Assad's forces have made swift gains since their offensive against Aleppo -- once Syria's commercial powerhouse -- began on November 15.
Tens of thousands of civilians have streamed out of the city's east, and Russia has renewed calls for humanitarian corridors so aid can enter and desperate residents can leave.
"After the recent advances, the regime is comfortably in control of half of former rebel territory in the city's east," he told AFP.
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The loss of east Aleppo -- a rebel stronghold since 2012 -- would be the biggest blow to Syria's opposition in more than five years.
Earlier today, anti-government fighters had successfully rolled back regime gains in Sheikh Saeed on Aleppo's southeastern outskirts.
Sheikh Saeed borders the last remaining parts of Aleppo still in rebel hands -- a collection of densely populated residential neighbourhoods where thousands have sought refuge from advancing regime forces.
It said four civilians were killed in rebel rocket fire on government-held areas, bringing to 59 the civilian toll in the city's west.
More than 300 civilians, including dozens of children, have been killed in east Aleppo since the offensive began, according to the Observatory.
Intermittent clashes today rocked residential buildings on Aleppo's eastern edges, as advancing regime forces seek to secure the road towards the airport.
AFP's correspondent in east Aleppo said ferocious clashes could be heard in the Tariq al-Bab district, where regime forces advanced yesterday.
Vegetable stalls -- empty for months because of a devastating government siege -- now lay shattered by heavy artillery fire.
The escalating violence has been met with international outrage, including a UN warning that east Aleppo could become "a giant graveyard".
Moscow has proposed setting up four humanitarian corridors into east Aleppo.