Syrian rebels were bracing yesterday 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 yesterday.

“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 yesterday or today,” 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.

Assad ‘preparing fresh slaughter’- There was a chorus of international concern over Aleppo, with a number of powers expressing fears of yet another massacre In late May, at least 108 people were killed near the central town of Houla, the United Nations said. On July 12, regime forces killed more than 150 people in the central village of Treimsa, including dozens of rebels, the Observatory said.

The opposition and part of the international community call it a massacre, with each side in the conflict blaming the other.

French foreign ministry spokesman Bernard Valero told AFP Friday that “with the build-up of heavy weapons around Aleppo, Assad is preparing to carry out a fresh slaughter of his own people.” Noting the massing of forces, US State Department spokesman Victoria Nuland said “this is the concern that we will see a massacre in Aleppo, and that’s what the regime appears to be lining up for.

“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.”

Valero said France shared US concerns over the rapidly deteriorating situation, and called on Assad to end the violence and step down. “Our message is that Assad must go,” he said. In Geneva, UN human rights chief Navi Pillay spoke of atrocities in the regime’s ongoing battle to cleanse Damascus of rebel elements and said Aleppo was also at risk. “I have been receiving as yet unconfirmed reports of atrocities, including extra-judicial killings and shootings of civilians by snipers” in Damascus, Pillay said.

“It goes without saying that the increasing use of heavy weapons, tanks, attack helicopters and-reportedly-even jet fighters in urban areas has already caused many civilian casualties and is putting many more at grave risk. “All this, taken along with the reported build-up of forces in and around Aleppo, bodes ill for the people of that city,” she said.

British Foreign Secretary William Hague expressed fears that “this utterly unacceptable escalation of the conflict could lead to a devastating loss of civilian life and a humanitarian disaster.”

For his part, former head of the troubled UN observer mission 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 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.” “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.” As the fighting raged, a lawmaker from Aleppo, Ikhlas Badawi, defected, according to opposition Syrian National Council member Samir Nashhar. 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. 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.