Activists confirm Jabhat al-Nusra take strategic towns after fierce five days of fighting with government troops.
The UN special envoy to Syria is travelling to Damascus to try to reduce the fighting which has intensified in Aleppo, where rebels claim to have killed 300 government soldiers in the past week.
Staffan de Mistura, who is on his fourth trip to the country since July and is expected to arrive in the capital on Monday, says he has received some assurances from the government that it will stop its aerial bombardment of Aleppo for six weeks.
Mistura has not yet received agreement from opposition groups and in a recent interview with Al Jazeera’s Diplomatic Editor James Bays said he knew many believed the odds were against the success of his initiative.
He is expected to travel to Istanbul following his visit to Syria to talk to opposition groups.
“My chances, I hope are not super slim, because that would in a way would also reflect the chances of the Syrian people to see hope at the end of this tunnel,” Mistura said.
“The one thing I can tell them is that the UN will never give up.”
Mistura’s plans involve a halt to the aerial bombardment and attacks using heavy weapons across the city of Aleppo, our correspondent reported.
He also aims to completely freeze all fighting on the ground in one district of Aleppo – Salaheddine in the southwest of the city.
UN humanitarian chief Valerie Amos said she was hoping Mistura’s plan would allow access for humanitarian aid workers in Aleppo.
“We are working very closely together because of course if he’s able to do what he’d like to do which is to deescalate the violence, that’s good for us, because it means that there are more places that we can get to,” Amos said.
“He’s trying to, once that violence is deescalated, to focus on trying to bring some normalcy to Aleppo, trying to get people to restart their livelihoods.”
The overall conflict in Syria has killed at least 220,000 people and sent more than 3.8 million people fleeing the country.