The attacks in Aleppo, which involved the shelling of government-held neighbourhoods and intense street fighting, came just days after the advance by the government side towards the Castello Road.
Rebels were relying on the road for supplies and access. Its severing effectively put opposition-held areas under siege.
The city, Syria's largest before the war, has been a main battleground in the country's five-year unrests.
Its capture would be a strategic prize for the Damascus government, which controls the major population centers in western Syria, Reuters report.
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