Syrian government forces have launched a counter-offensive under the cover of air strikes in an attempt to regain control of areas they had lost to insurgents the day before in Aleppo, activists and state media said.
The offensive came a day after Syrian rebels launched a broad ground attack aiming to break a weeks-long government siege on the eastern rebel-held neighbourhoods of Syria's largest city.
The insurgents were able to capture much of the western neighbourhood of Assad, where Saturday's fighting was concentrated, according to the Syrian army and the Britain-based Syrian Observatory for Human Rights.
The observatory said the new offensive by Syrian troops and their allies was ongoing under the cover of Russian and Syrian air strikes.
The group said the fighting and air strikes are mostly on Aleppo's western and southern edges.
The Syrian army command said troops and their allies are pounding insurgent positions with artillery shells and rockets, adding that "all kinds of weapons" are being used in the fighting in the Assad neighbourhood.
The Aleppo Media Centre, an activist collective, reported air strikes and artillery shelling of areas near Aleppo.
Syrian state media said rebels shelled government-held western neighbourhoods of Aleppo on Saturday morning, wounding at least six people, including a young girl.
Rebel shelling of Aleppo on Friday killed 15 and wounded more than 100.
On Friday, insurgents including members of Fatah al-Sham and the ultraconservative Ajnad al-Sham and Ahrar al-Sham militias took advantage of cloudy and rainy weather to attack government positions. On Saturday the weather was better, according to residents.
"There are ongoing clashes," said opposition activist Baraa al-Halaby, from besieged east Aleppo, adding that the fighting is far from them but explosions could be clearly heard in the city.
East Aleppo has been subjected to a ferocious campaign of aerial attacks by Russian and Syrian government war planes, and hundreds of people have been killed in recent weeks, according to opposition activists and trapped residents.
The new offensive by insurgents is the second attempt to break the government's siege of Aleppo's opposition-held eastern districts, where the UN estimates 275,000 people are trapped.
UN special envoy Staffan De Mistura has estimated 8,000 of them are rebel fighters, and no more than 900 of them affiliated with Fatah al-Sham.
Syrian and Russian officials have said that no ceasefire is possible as long as Fatah al-Sham remains allied and intertwined with other rebel forces.
Aleppo is the current focal point of the war.
President Bashar Assad has said he is determined to retake the country's largest city and former commercial capital.