A major offensive has been launched by Syrian rebels led by Islamist groups to take full control of the war-torn country’s northern city of Aleppo, the Syrian Observatory for Human Rights (SOHR) said Thursday. The fall of Aleppo, which is Syria’s major commercial hub, would restrict President Bashar Assad’s control to a territory stretching from the capital city of Damascus to the Mediterranean coast.

According to SOHR, rebels fired hundreds of rockets and missiles into government-controlled areas of Aleppo on Thursday, killing at least four civilians and wounding more than 70. The Syrian military also returned fire and launched airstrikes in western Aleppo, in which at least five rebel fighters were killed, the BBC reported, citing SOHR.

The offensive was launched “in order to support the Muslims in Aleppo and to take revenge [for] the weak from among the children and women whose families were raped and killed unjustly,” the insurgents, who include al-Qaeda's Syrian offshoot Nusra Front and a dozen other rebel groups, said in a statement obtained by SOHR.

The rebels said that they have formed a new alliance, called “Ansar al- Shari’ah,” which aims “to liberate the city of Aleppo and its countryside and to endeavor with the other factions to develop a joint charter to govern Aleppo after liberation according to Islam[ic] rules.”

Much of the fighting occurred on the Jamiyat al Zahra frontline, a key region which has long been sought by rebels, as it would allow them to reach government-held areas and disrupt army supply lines, Reuters reported, citing Rami Abdul Rahman, the head of SOHR.

Control of Aleppo is already divided between the government and rebel forces -- with the government in control of the western region of the city, and rebels in control of the east, which they seized in 2012.

Last month, rebels shelled Aleppo, killed at least 23 people, according to Syrian state-run television. SOHR also reported the incident, saying nearly 250 shells hit several neighborhoods in the city.