Fierce clashes raged overnight on the edges of Syria's second city Aleppo after a major rebel offensive, an AFP correspondent said Wednesday.
A coalition of opposition groups fighting under the name "Fatah Halab" -- or Aleppo Conquest -- launched the assault on President Bashar al-Assad's forces with a tunnel bomb on Tuesday.
Rebel fighters initially advanced from the western suburbs of the city into government-held districts but were pushed back by Wednesday morning, the Syrian Observatory for Human Rights said.
Observatory head Rami Abdel Rahman said the fighting was "the most violent in Aleppo in over a year."
The Britain-based monitor did not have an immediate count for combatants killed.
The intense clashes lasted through the night, with heavy artillery shelling and air strikes heard throughout the battered metropolis, an AFP correspondent in the city said.
Air strikes on rebel-controlled eastern neighbourhoods subsided as government warplanes focused on the fight in the west, he said.
By the morning, the clashes had quieted but weary residents were not expecting calm for long.
"I don't think the air strikes will stop, because the decision to stop firing isn't in Assad's hands -- it's in the hands of his Russian ally," said Mahmoud Sendeh, a 26-year-old activist in Aleppo's eastern districts.
"Up until now, it looks like Russia doesn't want calm to return to Aleppo," Sendeh told AFP.
Nearly two weeks of renewed fighting in Aleppo have left more than 270 civilians dead.
On Wednesday morning, three civilians were killed and one person wounded when rebel rocket fire crashed into a regime-held neighbourhood, state news agency SANA said.
Washington and Moscow are working together to include Aleppo in a so-called "regime of silence" -- a freeze in fighting -- aimed at bolstering a broader truce in place since February 27.
Since 2012, Aleppo city has been divided by a front line separating government forces in the west from rebel groups in the east.
But on the outskirts of the city, the inverse is true: rebel groups have nearly encircled the western half of Aleppo city, as regime forces have surrounded the eastern sides.
Rebel groups have tried to push in from the western suburbs into government-held territory, including in a major assault a year ago that also began with a huge tunnel explosion.
The UN Security Council is to hold urgent talks on the crisis on Wednesday after a call to action by France and Britain.
Also on Wednesday, German Foreign Minister Frank-Walter Steinmeier was to meet UN Syria envoy Staffan de Mistura, Syria's main opposition leader Riad Hijab and France's top diplomat Jean-Marc Ayrault in Berlin to press for peace.
More than 270,000 people have been killed and millions have been forced to flee since Syria's conflict erupted in March 2011.