The Syrian army launched a fight back against rebels in Aleppo on Saturday, amid concern among Western governments about reprisals against the civilian population of the country's second city.
Troops backed by tanks and helicopter gunships, which had been massing for the past two days, moved on southwestern districts of the commercial hub, where rebel fighters concentrated their forces when they seized much of the northern city on July 20.
Artillery pounded Salaheddin and other rebel neighbourhoods from 8 am (0500 GMT) as ground troops made their advance, an AFP correspondent reported.
Trapped civilians crowded into basements, seeking refuge from the intense bombardment.
"You can say that the fightback has begun," the head of the Britain-based Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP.
"The fiercest clashes of the uprising are taking place in several neighbourhoods of the city," he said.
Pro-government media had warned that the "mother of all battles" loomed in Aleppo as the government moved to reassert its authority after recapturing rebels' districts of the capital earlier in the week.
"Aleppo will be the last battle waged by the Syrian army to crush the terrorists and, after that, Syria will emerge from the crisis," the Al-Watan newspaper said.
Both sides acknowledged that casualties were likely to be high as the more than 16-month uprising comes to a head.
"Rebels are stationed in narrow streets, in which fighting will be difficult," a regime security official told AFP.
The opposition fighters had been holding their fire in readiness for the threatened counter-offensive, the AFP correspondent said.
But their small arms and rocket-propelled grenades were little match for the heavy armour of the Syrian army.
Three rebel fighters were killed in Aleppo on Saturday before the launch of the army's fightback, the Observatory said.
Five rebels and four civilians were killed in and around Damascus, and one civilian died in Daraa, south of the capital, cradle of the uprising against President Bashar al-Assad's rule which broke out in March last year.
On Friday, the rebels captured 150 soldiers and pro-government militiamen in Aleppo and in the northwestern province of Idlib on the Turkish border, the Observatory said.
Turkey, which has given refuge to defecting army officers who have formed the kernel of the rebel Free Syrian Army as well as tens of thousands of fleeing civilians, warned it could "not remain an observer" as the violence raged across its southern border.
"There is a regime there that kills and massacres its own people," said Turkish Prime Minister Recep Tayyip Erdogan.
"We must do what we can together in the United Nations Security Council, and also in the Organisation of Islamic Cooperation and the Arab League, to make sure that we can make some important progress in trying to avert this appalling situation."
UN chief Ban Ki-moon called on Damascus not to press ahead with the threatened assault on the country's second city.
"I'm seriously concerned by the escalating violence in Aleppo," Ban said. "I urge the Syrian government to halt the offensive. The violence from both sides must stop for the sake of suffering civilians."
British Prime Minister David Cameron said there were "very real concerns that we have that the Syrian regime is about to carry out some truly appalling acts around and in the city of Aleppo."
Italy called for "maximum pressure" on Assad's government to prevent further killings.
French foreign ministry spokesman Bernard Valero told AFP that "with the build-up of heavy weapons around Aleppo, Assad is preparing to carry out a fresh slaughter of his own people."
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, the Observatory said.
The former head of the troubled UN observer mission in Syria, Robert Mood, warned that even Assad's departure from office might not bring an end to civil war amid the sectarian tensions sparked by his Alawite minority's long domination of the Sunni Muslim majority.
"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," the veteran peacekeeper said.
"The situation could even get worse."
US ambassador Robert Ford sought to allay the fears of Alawites and other minority groups who have manned the militias that have been one of the mainstays of the regime.
"There are some who are uncertain of the future and fear retaliation because of the community to which they belong," said Ford, who returned to Washington in February after the mounting violence prompted the closure of the US embassy.
"Neither a community nor an ethnicity must be blamed for the actions of individuals in the regime," he said in a posting on his Facebook page.