The battle is one of the most important of the 17-month-old uprising. With a population of about 3 million, Aleppo is Syria's largest city and commercial hub, a key pillar of support for President Bashar Assad's regime.
The rebels controlled several neighbourhoods but were facing reports of troops and tanks massing outside the city. The nonstop fighting in Aleppo has already claimed the lives of at least 145 rebels and civilians in the last six days, according to the Britain-based Syrian Observatory for Human Rights.
UN High Commissioner for Human Rights Navi Pillay said reports are coming out of the capital, Damascus, of extra-judicial killings and shootings of civilians during fighting in that city's suburbs. Expressing deep alarm at the situation, Pillay said the report "bodes ill for the people" of Aleppo."
Pillay said she believes President Bashar Assad's regime and opposition forces are both committing crimes against humanity and war crimes.
Pillay has been following the conflict between President Bashar al-Assad's regime and opposition forces.
"I have been receiving as yet unconfirmed reports of atrocities, including extra-judicial killings and shooting of civilians by snipers, that took place during the recent fighting in various suburbs of Damascus," Pillay said.
"And 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."
The UN rights chief also contended that a "discernable pattern has emerged" in how the Syrian regime forces move in on a targeted city or village.
Pillay said the pattern suggested that regime forces cut off water, electricity and food supplies after surrounding a village or district. That is followed by "intense shelling and bombardment by a variety of weaponry," which reportedly includes air support from attack helicopters and jets. Tanks move in and ground forces move door-to-door, she said, adding that reports suggest some people are detained, while others are executed.
"The bodies of those executed or otherwise killed are then sometimes burned or taken away," she said.
The uprising against Assad, which began peacefully in March 2011, has become increasingly violent.
Gen. Robert Mood, the outgoing head of the UN monitoring mission, , in his opinion, it's "only a matter of time" before the regime falls.
"Every time there are 15 people killed in a village, 500 additional sympathizers are mobilized, roughly 100 of whom are fighters," said Mood, who has been replaced by Lieutenant General Babacar Gaye, a Senegalese military officer who has a mandate of only 30 days.
Mood, whose three-month mission in Damascus ended last week, also said the situation in Syria is likely to remain unstable even if Assad's government steps down.
"That could easily be the start of a situation that is way worse," he said.
The UN recently said about half of the 300 observers in Syria had been sent home.
Rebel fighters criticized
Opposition fighters were also criticized by Pillay, who said she has received more reports of rebel fighters torturing or executing prisoners.
"Murder and willful killing, whether committed by government or opposition forces, may constitute crimes against humanity or war crimes. Torture, likewise, is prohibited under all circumstances," she said.
Aleppo is not far from the border with Turkey, which, like neighbouring Iraq, Jordan and Lebanon, is hosting a growing number of refugees fleeing the violence in Syria.
The UN Refugee Agency said on Twitter Friday that roughly 1,400 people crossed from Syria to Jordan overnight, saying it was the most people to cross that border in one night.
Turkey's state-run agency said Friday a Syrian legislator from the northern city of Aleppo has fled to Turkey and warned that Syria was preparing for a massive offensive on cities where rebels are fighting government forces.
The Anadolu agency said Ikhlas Badawi has defected in protest of the Syrian regime's "violence against the people." She would be the first member of Syria's parliament to defect.
John Ray, a journalist who spent several days reporting for ITV in and around Aleppo, told CBC's As It Happens Thursday that there was a "heck of a fight going on" in the outskirts of the city Wednesday night through to early Thursday.
Ray, who was with a detachment of Free Syria Army fighters, said there were shells whizzing overhead and the constant rattle of machine-gun fire.
The forces at Assad's disposal appeared to be "far greater" than anything the rebels had at hand, Ray said Thursday from Turkey.
On Thursday, U.S. State Department spokeswoman Victoria Nuland said the U.S. had "grave concerns" about the situation in Damascus and the clashes in the densely populated commercial hub of Aleppo.
Nuland said Aleppo has been "bombarded by Syrian fighter jets in the latest desperate effort of the Assad regime to hold onto control" and added there are "credible reports" of columns of tanks preparing to attack the city.