Since July 17, President Bashar al-Assad's forces have surrounded rebel-held districts of Aleppo city, one of the main front lines in the conflict ravaging the country since 2011.
Loyalists forces cut the Castello Road, the main supply line into rebel-held neighbourhoods in the north of the city.
Now insurgents have attacked from the south, a region divided between loyalists backed by Iranian fighters and Hezbollah on the one hand, and Syrian and foreign jihadists allied with rebel groups on the other.
On Sunday, Islamist groups such as the influential Ahrar al-Sham and jihadists including from the former Al-Nusra Front -- rebranded Jabhat Fateh al-Sham after breaking from Al-Qaeda -- said they had begun a battle to try to reopen a new supply route.
Fateh al-Sham launched two car bomb attacks against regime positions in suburban Rashidin in southwestern Aleppo and fighting also raged in the early evening, the Syrian Observatory of Human Rights said.
Three children were among 11 civilians killed in rebel rocket attacks launched from Rashidin on the government-controlled district of Hamdaniyeh in western Aleppo, the Observatory said.
Other attacks focused on southern parts of the city towards the regime-controlled suburb of Ramussa, the Britain-based monitor reported.
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"It will be a long and difficult battle," said Observatory chief Rami Abdel Rahman.
"The army is supported by a large number of Iranians and fighters from Hezbollah, not to mention the Russian planes," he said.
Forces from Lebanon's Shiite group have been fighting alongside Assad's men in Syria for years, and Russia at the end of September last year began a campaign of air strikes in support of loyalist fighters.
In Aleppo city itself, regime forces bombarded rebel-held districts Sunday despite the announcement by Damascus and Moscow of humanitarian corridors to allow civilians and rebels ready to surrender to leave.
On Saturday, government media reported that dozens of civilians and rebels had left besieged eastern Aleppo through humanitarian corridors, but residents there and rebels dismissed the claims as "lies".
Elsewhere, at least nine civilians were killed Sunday in an air strike that hit a makeshift hospital at Jassem in the southern province of Daraa.
The International Rescue Committee, which supported the facility, called on the UN Security Council "to act in defence of the most basic principles of the UN".
"The bombing of hospitals is never justified. All those involved must be held to account," said IRC chief David Miliband in a statement.