Syrian security forces have stepped up their assault on rebel positions in the northwestern city of Aleppo, the country's commercial hub.

Opposition activists say troops with tanks and helicopter gunships pounded rebel-controlled neighborhoods on Saturday. The Syrian Observatory for Human Rights says at least 12 people have been killed in Saturday's fighting, including 10 soldiers.

Pro-government media have warned that what they describe as the “mother of all battles” looms in Aleppo.

Meanwhile, Russia warned a “tragedy” was imminent in Aleppo, but said foreign support of rebels would lead to “more blood.” Foreign Minister Sergei Lavrov also said the Syrian government could not be expected to willingly give in to the opposition.

Nearly three million people live in Aleppo, which had mostly stayed out of the conflict until recent weeks.