A rebel alliance and Syrian government forces have been locked in a battle in and around Aleppo city with President Bashar al-Assad's regime seeking to repel the major offensive with heavy air strikes.
Thursday's attack, the most intense rebel offensive in Aleppo in three years, aimed to build on recent advances against Assad by an array of groups fighting on separate fronts, including the Islamic State of Iraq and the Levant (ISIL) group and rebels backed by his regional foes.
A rebel alliance including the al-Qaeda-linked Nusra Front and the Islamist Ahrar al-Sham said they had set up a joint operations room to run the offensive to "liberate" Aleppo and later govern it according to Islamic law.
One Aleppo resident, a 23-year-old student who gave her name as Sahar, said fighting had been "intensive".
"We are used to the sound of explosions but yesterday there were so many. We heard the blasts but because they were coming from everywhere we didn't know where the shells were falling," she told the AFP news agency by telephone.
The UK-based Syrian Observatory for Human Rights, a group monitoring the war, said the fighting raged into the early hours of Friday.
Rami Abdelrahman, director of the group, said the rebel forces had seized some buildings from government control on the northwestern city outskirts of Jamiyat al-Zahra, but the advance was not of strategic importance.
At least 35 rebels were killed in that area, including a dozen Syrians and many others of Central Asian origin, Abdelrahman said. The Syrian war has drawn foreign fighters from across the Muslim world.
RELATED: What's left of Syria?
Syrian state television said the army had "foiled attempts to infiltrate Aleppo on several fronts, killing more than 100 terrorists" - the regime's standard term for all rebel groups.
A Syrian military source said the air force and artillery had been used to target the rebels, whom he said had used heavy weapons in their attack.
Air strikes were also reported near the town of Azaz in the north of Aleppo province, just over the border from Turkey.
Aleppo, 50km south of the Turkish border, was Syria's most populous city before the country's descent into civil war. It has been partitioned into zones of government and rebel control since 2012.
Aleppo is of vital importance to Assad, and losing it would further entrench a de facto partition of Syria between western areas still governed from Damascus and the rest of the country run by a patchwork of militias.