Rebel groups including the powerful Ahrar al-Sham faction and former Al-Qaeda affiliate Fateh al-Sham Front fired waves of rockets into government-held western Aleppo, killing at least 15 civilians, a monitor said.
The rebels also targeted government positions east of Aleppo city and in the coastal province of Latakia, including the Hmeimim military base that is used by Russian forces allied with the regime.
The assault comes more than three months into a government siege of eastern Aleppo, where over 250,000 people live, and several weeks after the army began an operation to retake the rebel east.
Rebel groups "announce the start of the battle to break the siege of Aleppo," said Abu Yusef Muhajir, a military commander and spokesman for Ahrar al-Sham.
The assault "will end the regime occupation of western Aleppo and break the siege on the people trapped inside," he told AFP.
"The breaking of the siege is inevitable," said Yasser al-Yusef, a member of the political office of the Nureddine al-Zinki rebel group.
"We will protect the civilians and schools and hospitals from Russian attacks and bring our people food and medicine," he said.
- 'God is greatest' -
The Syrian Observatory for Human Rights, a Britain-based monitor, said at least 15 civilians, including two children, had been killed, and more than 100 wounded in rebel fire on western Aleppo.
The monitor reported fierce clashes on multiple front on the western and southern outskirts of western Aleppo, with three suicide car bombs targeting a checkpoint in the Dahiyet al-Assad neighbourhood.
It had no immediate toll in the clashes or bomb attack.
An AFP correspondent in east Aleppo said the assault had boosted the mood in rebel-held districts, with mosques broadcasting "God is greatest" from loudspeakers to hail the battle.
He said residents were burning tyres, sending smoke up over the city in a bid to provide cover for rebel forces from government and Russian warplanes.
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The Observatory said rebel forces had also fired dozens of rockets at the Nairab military airport and Aleppo international airport, both east of Aleppo city and under government control.
And rebels also fired rockets from Idlib province into the government stronghold of Latakia, killing one person and wounding six.
The rocket fire hit near the Hmeimim military airport, as well as near Qardaha, the ancestral village of President Bashar al-Assad, the monitor said.
- 'A kill zone' -
Syrian state television reported the assault, saying "the army has foiled an attempt by terrorists to attack Aleppo city from several axes with suicide bomb attacks and has inflicted losses on them."
"Terrorist groups have made no advances and clashes are continuing," it added.
State news agency SANA said government planes were carrying out airstrikes south and west of Aleppo.
Once Syria's economic powerhouse, Aleppo has been ravaged by the conflict that began in March 2011 with anti-government protests and has since killed over 300,000 people.
Aleppo has been divided between government control in the west and rebel control in the east since mid-2012, and in September the army announced an operation to recapture the whole city.
The assault, backed by Russian forces, has killed hundreds of civilians and destroyed infrastructure including hospitals, prompting international outrage.
The UN's aid chief Stephen O'Brien this week said Aleppo had become "a kill zone", adding that "nothing is actually happening to stop the war, stop the suffering."
Last week, Russia implemented a three-day "humanitarian truce" intended to allow civilians and surrendering rebels to exit the east through passages to western neighbourhoods.
But few left, and a UN plan to evacuate the wounded failed because security could not be guaranteed.
Russia says it has not bombed Aleppo since October 18, and accuses rebel groups of preventing civilians from leaving.
On Friday, Syria's foreign minister was in Moscow meeting with his counterparts from key allies Russia and Iran.