Fighting on an "unprecedented" scale on Friday shook Syria's second city Aleppo, where rebels have declared a decisive battle, residents and the Syrian Observatory for Human Rights reported.

"The fighting is unprecedented and has not stopped since Thursday. The clashes used to be limited to one or two blocks of a district, but now the fighting is on several fronts," said Observatory director Rami Abdel Rahman.

Residents in the central districts of Sulimaniyeh and Sayyid Ali, previously spared the worst of fighting, also told AFP that the violence and mortar fire from rebels was "unprecedented".

"The sound from the fighting and the gunfire has been non-stop. Everyone is terrified. I have never heard anything like this before," said a 30-year-old resident of Sulimaniyeh who only identified himself as Ziad.

The rebels, who have been vastly outgunned by forces loyal to President Bashar al-Assad throughout the more than 18-month conflict, declared an all-out assault for the northern city on Thursday.

And an AFP correspondent at the scene counted about 16 mortar shots from 5:00 pm (1400 GMT) to 7:30 pm, with a shot about every 15 minutes in three army-controlled areas, including Sulimaniyeh and Sayyid Ali.

"This is the first time I have seen something like this in Sayyid Ali. Normally there are two or three mortars. But last night the intensity was unprecedented," another resident told AFP on condition of anonymity.

"One of the mortars hit a residential building and killed four people from the same family, including an old man and a young child. We tried to carry them away to bring them to the hospital but they were already dead. So we left to help the others," added the resident.

"It was a horrible scene in the street. A whole crowd was trying to help. People brought cars to take the wounded to the hospital. There were children and whole families because it's a civilian area."

According to the correspondent, clashes were also ongoing in Sakhur in the east, where army shelling attacks would be followed by five minutes of machinegun fire and a brief stop, then the shelling would begin again.

The correspondent said that the sky in Aleppo has been clouded with dust and smoke since Thursday night until the morning because of the intense bombing and shelling.

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