Fierce fighting erupted in the Syrian city of Aleppo after a coordinated attack by a coalition of rebel groups launched major offensive against positions held by the Assad regime.

The Syrian Observatory for Human Rights (SOHR) said the rebels clashed with regime forces backed by National Defence Forces (NDF) militias and Lebanese Shia militant group Hezbollah.

The rebels, made up of various Islamist and secular groups including the al-Qaeda affiliate Jabhat al-Nusra and organised from the Ansar al-Sharia operations room, fired hundreds of rockets and shells into at least seven government-held areas late on Thursday.

The Assad regime responded the to attack with heavy artillery bombardment and dropping barrel bombs on rebel-held areas of the city, and the state-run Sana news agency reports that the rebel attack has been repelled with 100 “terrorists” killed.

SOHR reports that nine civilians were killed in the clashes.

Aleppo, the economic centre of Syria has been divided between rebel and regime control since the early months of the Syrian civil war. Ground battles and air strikes in the city have left thousands dead and more than 60% of the Old City, a World heritage site, destroyed.

More than 230,000 people are believed to have been killed in the Syrian conflict since the uprising against President Bashar al-Assad began in March 2011. At least 11.5 million others, more than half of the country’s population, have fled their homes. According to the UNHCR, nearly 4 million Syrian refugees now live outside the country, mostly in Lebanon, Jordan, Turkey, and Iraq.