Columns of armed forces and armour have poured into Aleppo over the past two days with troops firing on a string of rebel neighbourhoods in the battle for control of the pivotal northern city.
The US showed heightened concern over the government offensive but rejected comparisons to a Libyan crackdown that triggered international intervention.
Amid widespread expectations that an increasingly desperate President Bashar al-Assad will pull out all the stops to secure Aleppo, residents of some areas of the commercial capital fled in fear of a massacre.
The redeployments were "almost complete," a Syrian security official said, while adding that "rebels are stationed in narrow streets, in which fighting will be difficult".
Colonel Abdel Jabbar al-Okaidi of the rebel Free Syrian Army (FSA) said "we expect a major offensive at any time".
Syrian Observatory for Human Rights director Rami Abdel Rahman said: "The question is, to what extent will regime troops resort to excessive use of force, because if this is the case, there will be hundreds of deaths."
Aleppo, Syria's second city, has been the scene of fierce fighting since rebels launched a major offensive on July 20 from rear bases across the nearby border with Turkey.
Clashes with rebels
The Observatory said helicopter gunships attacked the southwest of the city, in the Salaheddin, Bustan al-Qasr, Sukari, Al-Mashhad and Al-Azamiya neighbourhoods.
In Salaheddin, a rebel said residents were fleeing and confirmed helicopters had been firing on the district since daybreak. Troops were on the outskirts of the area and had not yet tried to enter.
Troops also clashed with rebels in the central district of Al-Jamaliya, adjacent to the historic old quarter, as well as in Mahatat Baghdad and in Saadallah al-Jabiri Square.
An AFP journalist said people were starting to go hungry in Aleppo because of a shortage of bread.
He also said the FSA was conserving its ammunition, especially rocket-propelled grenades, in anticipation of the expected tank assault.
The Observatory reported at least four deaths in Aleppo on Friday, among 119 people killed nationwide – 65 civilians, 35 soldiers, 18 rebels and one deserter.
Amid the skirmishes, the rebels captured 150 soldiers and militia members in Aleppo and the northwestern province of Idlib, the Britain-based Observatory said.
On Thursday, pro-government daily Al-Watan said Aleppo would be "the mother of all battles".
Fears of another massacre
"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.
A number of powers expressed fears of yet another massacre.
In late May, at least 108 people were killed near the central town of Houla, the UN said. On July 12, regime forces killed more than 150 people in the central village of Treimsa, including dozens of rebels, the Observatory said.
White House spokesperson Jay Carney rejected on Friday comparisons between Aleppo and Benghazi, the rebel-held Libyan city that was an early focus of the uprising against Muammar Gaddafi.
Gaddafi loyalists launched a major offensive against the Benghazi rebels, and were attacked by an international coalition including the US enforcing a UN-mandated no-fly zone.
"There were a broader array of issues that allowed for the kind of action that the US – you know, the international community, led by the US, was able to take in Libya," Carney said.
Turkey's Prime Minister Recep Tayyip Erdogan urged international action in Syria, saying it was not possible "to remain a spectator" to the regime's offensive on Aleppo.
Erdogan, speaking after meeting British Prime Minister David Cameron in London, urged joint action from the UN Security Council, the Organisation of Islamic Cooperation and the Arab League.
Italy called for "maximum pressure" on Assad to prevent further killings, while UN chief Ban Ki-moon urged his regime to stop its Aleppo offensive.
"I'm seriously concerned by the escalating violence in Aleppo," Ban said in London. "I urge the Syrian government to halt the offensive. The violence from both sides must stop for the sake of suffering civilians."
French foreign ministry spokesperson Bernard Valero said "with the build-up of heavy weapons around Aleppo, Assad is preparing to carry out a fresh slaughter of his own people."
His comments came as two Syrian opposition activists unfurled an enormous rebel flag at the base of the Eiffel Tower in Paris.
In Geneva, UN human rights chief Navi Pillay spoke of atrocities in the regime's ongoing battle to cleanse Damascus of rebels 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."
And the former head of the troubled UN observer mission, Major General Robert Mood, whose mandate to lead a 300-strong mission ended last week amid a sharp spike in violence, said Assad's fall was a matter of time.
"But will it fall in a week or in a year? That is a question I do not dare answer." – Sapa-AFP