Syrian rebel forces are bracing for the “mother of all battles” in Aleppo, as the former UN observer mission chief said President Bashar al-Assad’s fall was a matter of time.

Waves of troop reinforcements have been pouring into the northern city – Syria’s second largest – and a government security official told AFP the offensive feared by the rebels could come as early as today.

“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.

Earlier, helicopter gunships strafed a string of rebel neighbourhoods in the southwest of the city. Clashes also broke out in the Al-Jamaliya district, adjacent to Aleppo’s historic old quarter, a human rights watchdog said.

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

In Salaheddin, a rebel bastion in the southwest of the city, hundreds of opposition fighters were bracing for the counter-offensive threatened by the embattled Assad regime.

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

Four of the five roads between the city centre and the airport were under rebel control.

“The army’s reinforcements have arrived in Aleppo,” Colonel Abdel Jabbar al-Okaidi, a spokesman for the rebel Free Syrian Army, told AFP via Skype, adding they were backed by some 100 tanks.

“We expect a major offensive at any time.”

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.

The former head of the troubled UN observer mission in Syria, Major General Robert Mood, said Assad’s fall was a matter of time but that his exit might not end the civil war.

“Sooner or later, the regime will fall,” said the Norwegian general, whose mandate to lead a 300-strong mission ended last week amid a sharp spike in violence.

“But will it fall in a week or in a year? That is a question I do not dare answer,” he said.

The veteran peacekeeper cautioned that rebel success might not necessarily mean an end to the conflict.

“Many think that if Bashar al-Assad falls or that if he is given an honourable exit… the problem will be solved. That is an over-simplification one should be wary of,” Mood said.

“The situation could even get worse.”

Mood stepped down as head of the UN Supervision Mission in Syria last week as the Security Council gave it a “final” 30-day mandate in the face of the escalating violence.

Washington warned of the risks of a massacre by government troops in Aleppo, where rebel forces launched a major offensive on July 20 from their rear-bases across the nearby border with Turkey.

“This is the concern that we will see a massacre in Aleppo, and that’s what the regime appears to be lining up for,” said State Department spokeswoman Victoria Nuland.

“Our hearts are with the people of Aleppo. And again, this is another desperate attempt by a regime that is going down to try to maintain control, and we are greatly concerned about what they are capable of in Aleppo.”

As the fighting raged, a lawmaker from Aleppo, Ikhlas Badawi, defected, the exiled opposition said.

“She arrived yesterday (Thursday) in Turkey and she will be going to Qatar, which has agreed to receive her,” opposition Syrian National Council (SNC) member Samir Nashhar told AFP.

She is the fourth member of parliament to have defected since the uprising broke out in March last year.

No fewer than 27 generals have also defected to Turkey but they complain they have received little backing from the SNC which is largely composed of long-outlawed exile groups.

“We established the higher council with the army defectors in Turkey, but this council has not received adequate support from the SNC,” one of the defectors, Brigadier General Fayez Amr, told AFP.

There has also been a mounting exodus of civilian refugees across Syria’s borders in recent days.

Syrian troops opened fire on a group of civilians fleeing into neighbouring Jordan late on Thursday, killing a three-year-old child, Jordanian officials said.

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