Syrian army has launched ground and air attacks against rebels in parts of Aleppo, activists say.
Rebels with the Free Syrian Army (FSA) say they have repelled an army incursion and destroyed tanks, but there is no independent verification.
Western nations have warned of a potential massacre in Aleppo, Syria’s most populous city.
Early on Saturday morning, activists said Syrian tanks began moving in on south-western districts of the city.
They said the bombardment of rebel-held areas intensified in the early morning, with military aircraft overflying the city at low altitudes.
Many casualties have been reported, our correspondent says, and a steady stream of vehicles carrying families is leaving Aleppo.
Syrian state television said that rebels, having failed in Damascus, were now trying to turn Aleppo into a den for their terrorism.
The rebels say they have destroyed a number of tanks, but their claim cannot be independently verified.
The rebels are vastly outgunned and outmanned by forces loyal to Syrian President Bashar al-Assad.
Syrian army has launched ground and air attacks against rebels in parts of Aleppo
Activists have reported violent clashes around the Salah al-Din and Hamdanieh quarters near the centre of Aleppo.
An emergency call has gone out to doctors to come to Salah al-Din and help if they can, our correspondent says.
On Friday, the Red Crescent suspended some of its operations in Aleppo because of the heavy fighting.
Rebels had been stockpiling ammunition and medical supplies in preparation for the expected assault.
Both sides are braced for heavy casualties.
“Rebels are stationed in narrow streets, in which fighting will be difficult,” a government security official told the AFP news agency.
The Syrian Observatory for Human Rights said 160 people were killed across Syria on Friday.
The fighting comes after two weeks during which rebels made significant gains.
On 18 July, an attack at Syrian security headquarters in Damascus killed four senior officials, including the defence minister and President Bashar al-Assad’s brother-in-law.
The Free Syrian Army (FSA) took control of several parts of Damascus before being driven out by a government counter-offensive.
The rebels also seized several border crossings with Turkey and Iraq.
There has been fighting around Aleppo for the past week, with the government deploying fighter jets and helicopter gunships to beat back the rebels.
Until recently, Aleppo and Damascus had been relatively free of the violence that has wracked other parts of the country.
Earlier this week, thousands of government forces were moved from the border with Turkey to join fierce fighting in Aleppo, activists said.
On Friday, UN Secretary General Ban Ki-moon urged the Syrian government to halt its offensive and demanded a clear statement that chemical weapons would not be used under any circumstances.
Syria has implicitly acknowledged that it has chemical weapons but says it will not use them against its own people, only against foreign invaders.
The former head of the UN monitoring mission in Syria, Maj. Gen. Robert Mood, said it was “only a matter of time” until President Bashar al-Assad was ousted.
On Sunday, the Syrian Observatory for Human Rights reported that at least 19,106 people had been killed since March 2011. The UN said in May that at least 10,000 people had been killed.
Syria blames the violence on foreign-backed “armed terrorist gangs”.
In June, the Syrian government reported that 6,947 Syrians had died, including at least 3,211 civilians and 2,566 security forces personnel.
Robert Mood, the former head of the UN observer mission in Syria, says it is “only a matter of time” until President Bashar al-Assad’s government falls.
But Norwegian Maj. Gen. Robert Mood, who left Syria last week, said Bashar al-Assad’s fall would not necessarily mean an end to the 16-month-old conflict.
Syrian forces renewed their assault on the northern city of Aleppo, Syria’s most populous city, on Friday.
The US state department says it fears a massacre by Syrian government forces.
The pro-government al-Watan newspaper warned that the “mother of all battles” was about to start.
“In my opinion it is only a matter of time before a regime that is using such heavy military power and disproportional violence against the civilian population is going to fall,” Maj. Gen. Mood told the Reuters news agency.
Separately, UN High Commissioner for Human Rights Navi Pillay appealed to both sides to spare civilians, citing concerns of “the likelihood of an imminent major confrontation”.
Navi Pillay said she had received “as yet unconfirmed reports of atrocities, including extra-judicial killings and shooting of civilians by snipers” in Damascus.
Saying she had also received more reports of opposition fighters torturing or executing prisoners, Navi Pillay stated her belief that “crimes against humanity and war crimes have been, and continue to be, committed in Syria”.
Robert Mood, the former head of the UN observer mission in Syria, says it is only a matter of time until President Bashar al-Assad's government falls
An activist based in Fardos in Aleppo said at least 15 people had died on Friday morning during the military’s bombardment of a building.
“We have medical supplies but no doctors or equipment to treat the injured. The situation feels hopeless,” said the activist, identified only as Ramy.
“The people of Aleppo are not coping with this crisis. They are dying. It is a massacre. People can leave their homes and move around the city but who would really want to take the risk of being shot or bombed?”
He insisted that activists would continue to resist the government forces.
“Activists are prepared to engage in a guerrilla war, from street to street if necessary,” he said.
The Red Crescent has suspended some of its operations in Aleppo because of the heavy fighting.
Rebels have been stockpiling ammunition and medical supplies in preparation for the expected assault.
Syrian troops fired from helicopter gunships on south-western neighborhoods on Friday morning, the Syrian Observatory for Human Rights told the AFP news agency.
A convoy of tanks from Idlib province, near the border with Turkey, arrived in Aleppo overnight and was attacked by rebels, the Observatory said.
The US State Department said the deployment of tanks, helicopter gunships and fixed-winged aircraft around Aleppo suggested an attack was imminent.
But the US would not intervene, said spokeswoman Victoria Nuland, except by continuing to channel non-lethal assistance (such as communications equipment and medical supplies) to the rebels.
A Syrian MP from Aleppo has fled to Turkey, Turkey’s state-run Anatolia news agency says.
Ikhlas Badawi, a mother of six, said she was defecting in protest at the “violence against the people”.
Meanwhile, another defector, Gen. Manaf Tlas, has put himself forward as a possible figure to unite the fractious opposition.
In an interview with a Saudi newspaper, Asharq al-Awsat, he said: “I am discussing with… people outside Syria to reach a consensus with those inside.”
However, some in the opposition regard Gen. Manaf Tlas – who fled earlier this month – as a compromised figure too close to the government of President Bashar al-Assad.
For its part, Turkey has said it will not tolerate the creation of a Kurdish-run region in northern Syria.
This follows reports that Kurdish rebels in northern Iraq had formed an alliance with a Kurdish party across the border in Syria.
Turkey would strike against “terrorists” in northern Syria, warned Prime Minister Recep Tayyip Erdogan, in the same way it has attacked bases in northern Iraq used by militants linked to the Kurdistan People’s Party (PKK).
Turkey is concerned that the creation of a Kurdish authority in the north of Syria could provide a sanctuary to Kurdish rebels fighting for self-rule in Turkey’s southeast.