The Free Syrian Army has claimed more than 52 tanks have rolled into the district of Salah al Deen on the edge of Aleppo as government forces launch an assault to retake control of the city.
Activists say the fighting is concentrated in the southern and western suburbs.
There are numerous reports of heavy shelling and gunfights - there are also claims that the regime is using helicopter gunships in the assault.
Civilians caught up in the fighting are fleeing to northern parts of the city.
Human rights groups fear a massacre is unfolding.
Thousands of government troops have massed around the city over the last 48 hours.
The counter offensive by President Bashar al Assad's forces comes after several days of intense fighting in which the Free Syrian Army has taken control of a number of neighbourhoods.
Aleppo, the second city is Syria's largest; it is vital for the regime to keep it under its control as it is the commercial centre of the country and was until recently a bedrock of support for Mr Assad.
The FSA and other militia groups have been digging themselves in preparation for the fight.
A pro-Syrian government daily said that government forces were expecting the "mother of all battles".
The International Red Crescent has suspended its operations in Aleppo because of the escalating violence.
There have also been reports of fighting in other major cities around the country.
The volatile situation has caused more civilians to flee for their lives but reaching safety is hazardous.
An eight year old boy called Bilal has been buried in Jordan. His mother claims he was shot whilst trying to cross the border.
She said: "The Syrian army started shooting at us. We entered the Jordanian border, they rescued him but he was dead. He was shot in the head and the bullet left through his neck."
According to the UN, more than 120,000 people have now fled the fighting to neighbouring countries but that figure is expected to rise.
But there are signs the regime is weakening.
Another high-profile defector has fled to Turkey. The MP Ikhlas al-Badawi is now urging others to do the same.
He said: "I have crossed to Turkey and defected from this tyrannical regime.... because of the repression and savage torture against a nation demanding the minimum of rights."
Meanwhile, Foreign Secretary William Hague has met UN Secretary General Ban Ki-moon to discuss the crisis. He has called on Mr Assad to stop the fighting.
In a separate development, there has also been confirmation that Turkey, Qatar and Saudi Arabia are providing intelligence and weapons to the rebels.