Syrian rebels have begun a counter-attack in Aleppo, with heavy shelling of government-held areas after a weeks-long Russian-backed offensive against besieged districts held by insurgents.
The British-based Syrian Observatory for Human Rights says insurgents have set off several suicide car bombs on the western edge of the city.
A Syrian military source said on Friday an insurgent attack in that area had been thwarted. A state TV station said the army had destroyed four car bombs.
The rebels aim to break a siege imposed by government and allied militias with air support from Russia.
The rebel-held eastern districts of Aleppo have been subjected to fierce bombardment since the army declared an offensive to capture the area last month.
"There is a general call-up for anyone who can bear arms," a senior official in the Levant Front rebel group, which fights under the Free Syrian Army banner, told Reuters.
"The preparatory shelling started this morning."
The attack appeared mostly launched by rebels from outside the city against government forces.
An Ahrar al-Sham rebel spokesman also said in a social networking message an offensive on Aleppo had begun on Friday.
Factions involved in the attack include Free Syrian Army groups and Jaish al-Fatah, an alliance of Islamist factions, the Levant Front official said.
Grad rockets were launched at Aleppo's Nairab air base, said Zakaria Malahiji, head of the political office of the Aleppo-based Fastaqim rebel group, adding it was going to be "a big battle".
Heavy rebel bombardment, with more than 150 rockets and shells, struck districts on the southwest of the city, the Observatory reported.
It said more than 15 civilians had been killed and 100 wounded by rebel shelling of government-held western Aleppo. State media reported five civilians killed.
The Observatory also said Grad surface-to-surface rockets had struck Nairab air base and also locations around the Hmeimim air base, near Latakia.
