The newly-formed Syrian opposition launched a counter-attack against the Syrian regime and its allies on Friday in an attempt to break a week-long siege on eastern Aleppo, the opposition said.
The city has been divided for years between the regime-held western sector and opposition-held east.
The regime army and its allies laid siege to the eastern sector this summer and launched a new offensive in September that medics say has killed hundreds.
The Syrian Observatory for Human Rights (SOHR), a British-based war monitor, said more than 15 civilians had been killed and 100 wounded by opposition shelling of regime-held western Aleppo.
The assault, employing heavy shelling and suicide car bombs, was mainly focused on the city’s western edge by armed opposition fighters based in the countryside outside Aleppo.
It included Jabhat Fateh al-Sham, previously known as the Nusra Front, and groups fighting under the Free Syrian Army (FSA) banner.
A senior official of the newly-formed opposition said, "There is a general call-up for anyone who can bear arms."
SOHR said the opposition had gained most of Dahiyet al-Assad, a suburb in the southwest corner of the city.
The opposition launched Grad rockets at Aleppo's Nairab air base at the beginning of the assault according to the opposition's Zakaria Malahifji who added that it was going to be "a big battle".
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Another official said the extent of cooperation between the different rebel factions was unusual, and that the largest axis of attack was on the western edge of the city.
Aleppo has become the main theatre of conflict between Bashar al-Assad whose regime is backed by Iran, Russia and Shia militias, and the newly-formed opposition which is made up of rebel groups including those supported by Turkey, Gulf monarchies and the United States.
Syria's civil war, now in its sixth year, has killed close to a half a million people, displaced half the country's pre-war population, dragged in regional and global powers and caused a refugee crisis in the Middle East and Europe.