BEIRUT (AP) - An anti-government activist in the Syrian city of Aleppo says government forces have not managed to retake any of the city's neighborhoods seized by rebels a week ago.

Government tanks backed by attack helicopters swept into Syria's commercial hub today. Reinforcements have been pouring into the area in recent days.

The assault is seen as one of the most important battles of a 17-month uprising against President Bashar Assad's regime. It has raised fears among activists and the international community that a new massacre could be looming.

Russia is adding its voice to mounting chorus of international concern. Foreign Minister Sergei Lavrov says it's unrealistic to expect the Syrian army to stand by while rebels take over major cities but a "tragedy" could be looming in Aleppo.

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