A member of the Free Syrian Army sits beside his weapon in Azzaz, Aleppo province.

Syrian rebels were bracing Friday for the “mother of all battles” in Aleppo, with European powers fearing a “slaughter” was imminent and calling for maximum pressure to prevent it.

As reinforcements poured into the city in what has been said could be a potential watershed in the 16-month conflict, the former UN observer mission chief said President Bashar al-Assad's fall was a matter of time.

Waves of troops have been pouring into Aleppo, Syria's largest city and strategic commercial hub, and a government security official told AFP the offensive feared by the rebels could come as early as Friday.

“The special forces were deployed on Wednesday and Thursday on the edges of the city, and more troops have arrived to take part in a generalised counter-offensive on Friday or Saturday,” the source said.

Fighting for control of the city is now in its ninth day.

In the rebel stronghold district of Salaheddin, hundreds of rebels were bracing for the threatened counter-offensive.

An AFP photographer saw improvised barriers made up of sandbags and even a bus thrown up across the street, as well as makeshift clinics set up inside schools and mosques.

“We expect a major offensive at any time,” Colonel Abdel Jabbar al-Okaidi, a spokesman for the rebel Free Syrian Army, told AFP via Skype.

A rebel fighter in Aleppo, reached by telephone, said gunships had been firing on rebel areas since 6:00 am (0300 GMT). Troops were on the outskirts but had not yet tried to enter.

Clashes also broke out in Al-Jamaliya district, adjacent to Aleppo's historic old quarter, a watchdog said.

Three people were killed in shelling of the southern district of Fardoss and one was shot dead in Maysaloon neighbourhood, said the Syrian Observatory for Human Rights.

The front page of Thursday's edition of pro-government daily Al-Watan carried the banner headline: “Aleppo, the mother of all battles.”

“Aleppo will be the last battle waged by the Syrian army to crush the terrorists and, after that, Syria will emerge from the crisis,” it said. - AFP