Syrian and Russian aircraft pounded rebel-held areas of Aleppo on Friday, a monitor said, after the army announced a new offensive aimed at retaking all of the divided second city.

At least 30 strikes hit the rebel-held east of the city during the night and early on Friday, killing at least three people, the Syrian Observatory for Human Rights said.

The Britain-based monitoring group said that dozens of people were wounded and that more dead were feared buried under the rubble.

Two civil defence centres were damaged in the bombardment, an AFP correspondent reported.

Russian warplanes were carrying out strikes alongside Syrian aircraft, Observatory chief Rami Abdel Rahman said.

"The Syrians are dropping barrel bombs and the Russian planes are launching strikes," he told AFP.

The raids came after the Syrian army announced late on Thursday that it was launching a new offensive to retake rebel-held parts of the city.

The Observatory said it was "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.

Source: AFP