The Syrian army has stepped up its assault on the country's largest city, pounding rebel-controlled neighborhoods with tanks and helicopter gunships.

Artillery rained down on the district of Salaheddin and other rebel neighborhoods in the city of Aleppo Saturday, as residents took refuge in basements or fled the city for the countryside.

Pro-government media have warned that what they describe as the “mother of all battles” looms in Aleppo. The U.S., Britain and the U.N. have voiced growing alarm of an imminent massacre there.

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