An AFP correspondent in the opposition-held east of the city reported intense bombardment both from the air and by ground artillery.
It came after the Syrian army announced late on Thursday that it was launching a new offensive to retake rebel-held parts of the city.
A military source said the bombardment was in preparation for a ground operation.
"We have begun reconnaissance, aerial and artillery bombardment," he told AFP.
"This could go on for hours or days before the ground operation starts. The timing of the ground operation will depend on the results of the strikes and the situation on the ground."
An army officer in Aleppo confirmed that the ground assault had yet to begin.
The Syrian Observatory for Human Rights reported at least 30 strikes on rebel-held districts during the night and early on Friday.
The Britain-based monitoring group said at least 10 people had been killed, among them two children, and dozens wounded.
It said more dead were feared buried under the rubble.
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The AFP correspondent said the scale of the destruction was the heaviest he had seen in years of fighting in the city and was overwhelming rescue teams.
He said two civil defence centres were among the buildings hit in the bombardment, reporting artillery barrages, barrel bombings by helicopters and strikes by fighter jets in quick succession.
Observatory chief Rami Abdel Rahman said Russian warplanes were taking part in the strikes alongside Syrian aircraft.
"The Syrians are dropping barrel bombs and the Russian planes are launching strikes," he told AFP.
He said it was the prelude to "a large-scale land offensive supported by Russian air strikes aimed at taking bit by bit the eastern sector of Aleppo and emptying it of its residents."
A truce deal hammered out between Russia and the United States briefly halted the violence earlier this month, but it collapsed after just a week without any of the promised deliveries of desperately needed relief supplies.
UN envoy Staffan de Mistura warned: "What is happening is Aleppo is under attack and everyone is going back to the conflict."
Aleppo was once Syria's commercial and industrial hub but has been ravaged by fighting and roughly divided between government control in the west and rebel control in the east since mid-2012.
Rebel districts have been under siege by the army for most of the past two months after troops overran the last supply lines.
More than 300,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.