Intense shelling and mortar rounds rocked Syria's northwestern city of Aleppo on Saturday as security forces stepped up assaults on rebel positions.

Opposition activists say troops with tanks and helicopter gunships have pounded neighborhoods with rebel strongholds.

The Syrian Observatory for Human Rights says at least 29 people were killed in Saturday's unrest in the commercial hub. The Britain-based opposition group says at least 90 people died in anti-government related unrest across Syria.

United Nations special envoy Kofi Annan on Saturday said he feared an “imminent battle” for Aleppo. Both France and Russia also echoed his concern about the mounting violence.

French President Francois Hollande renewed a call for U.N. to intercede, saying the role of U.N. Security Council member states is to intervene “as quickly as possible.”

Earlier, 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.