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The Aleppo office of Qatar-based TV broadcaster al-Jazeera was partially damaged as missiles rained down on rebel-held areas of the divided city on Friday, causing widespread destruction and at least 90 civilian casualties.

Al-Jazeera's Amr al-Halabi from city held back tears as he spoke about the intense bombardment, which included artillery barrages and barrel bombings by helicopters.

"Right now as I speak to you all there are four Russian warplanes in the sky heavily bombarding the city with seismic, cluster and phosphorus bombs," Halabi said.

"As I walked through the city all I saw were people crying… I am not here to deliver an emotional statement but the situation is catastrophic,"

"I can't go on what I've seen here is just too much."

Al-Jazeera on Friday that its bureau in the city had been partly destroyed by Russian bombing. The news agency its official operations inside Syria in 2011 following what it says was an intimidation campaign against its staff.

	Rescue teams have been overwhelmed by the bombing [Getty]

Syria's military threatened a ground offensive in Aleppo and pounded the city's rebel-held neighbourhoods with airstrikes on Friday, killing dozens, demolishing buildings and damaging a main water station.

White Helmets civil defence press officer, Ibrahim al-Hajj, told °®Âþµº on Friday that at least 90 civilians had been killed and hundreds more injured.

"These bombs that create earthquake-like shocks as they hit the ground are aimed at killing as many civilians as possible by collapsing buildings on top of their occupants," Hajj said.

"There are still around 20 bodies trapped under rubble. We are overwhelmed but are working to recover them."

He added that around 150 air raids struck various parts of the city, including three White Helmets' headquarters.

The bombardment came a day after the Syrian army announced an offensive to recapture east Aleppo, which has been held by the rebels since mid-2012 but has been surrounded by government forces since July.

The conflict in Syria has cost more than 300,000 lives and displaced over half the country's population since March 2011.