Syrian state media said on Monday the army and its allies had captured the al-Sakhour district from rebels in eastern Aleppo.
According to Reuters, the takeover risks splitting the area help by opposition in two.
According to the State TV, the army and its allies had seized the entire area and were working to clear it of mines.
It did not say whether the district was captured late on Sunday or early on Monday.
Meanwhile, rebel sources are yet to be reached.
“It is the biggest defeat for the opposition in Aleppo since 2012,” Rami Abdulrahman, Director of the Observatory, told Reuters.
“The opposition has lost more than third of the area it controlled in the city during the big advance,” he said.