Opposition social media showed videos of rebel tanks, armored personnel carriers, grad rockets and artillery firing from the southwest of the city, in an apparent attempt to break through Syrian government offenses from the outside of Aleppo.
"The whole city can hear the clashes, it is very huge," a resident in a government-controlled part of the city told CNN on condition of anonymity.
At least 15 people were killed and more than 100 others were wounded when hundreds of mortar rounds showered down on several residential neighborhoods in regime-held areas of western Aleppo, the UK-based monitoring group Syrian Observatory for Human Rights (SOHR) said in a statement.
Clashes are continuing in several areas on the outskirts of western Aleppo between Syrian forces and various rebel groups, including Jabhet Fateh al-Sham, according to SOHR.
"It is more than big," said Ismail Abdullah, a rescue worker in Aleppo's rebel held east, when asked about the fighting.
Russia's military has asked President Vladimir Putin for permission to resume airstrikes on the eastern part of the city after a lull in fighting last week, Russia's state news agency Tass reported Friday.
Jabhet Fateh al-Sham fighters carried out three suicide car bombings targeting regime forces in the southwestern outskirts of Aleppo, according to the SOHR, which said there were many killed and wounded from the bombings but could not provide an exact number yet.
Jabhet Fateh al-Sham -- formerly al-Nusra Front -- confirmed the offensive had begun on their Telegram channel. On Twitter, the al-Sham Legion rebel group announced that shelling on government areas was underway.
The rebel attack comes after Syrian government and allied Russian forces declared a unilateral ceasefire
last week, which brought several days of relative calm after weeks of intense aerial bombardment.
But the ceasefire effectively ended Sunday when warplanes resumed bombing the city, according to residents.
The United Nations blames both sides in the conflict for failing to take advantage of ceasefire to allow the evacuation of some of the hundreds of thousands of civilians believed to be trapped in eastern Aleppo.
The UN Under-Secretary-General for Humanitarian Affairs said on Wednesday that at least 400 people had been killed by Russian and Syrian government aerial bombardment of eastern Aleppo in less than a month.
Stephen O'Brien also told the UN Security Council that throughout October, rebel mortar strikes killed at least 100 people, including 17 women and 22 children.
CORRECTION: An earlier version of this story incorrectly suggested that the deaths of six children at a school were related to a new offensive launched by rebels Friday. The violence on the school occurred Thursday.