Rebels say they have taken control of more than "50 per cent" of Aleppo, Syria's largest city and commercial hub where opposition strongholds have been under attack for more than 10 days.
Despite Free Syrian Army (FSA) claims of a strengthening hold on the city, government forces continued on Friday to shell rebel-held areas and bomb them with advanced fighter jets, destroying many residential buildings.
The FSA claims to have consolidated most of its control in the city's east, while also maintaining a grip on central neighbourhoods including Salaheddin and Bab al-Hadid.
When a government security source was questioned about the lack of a ground offensive by the army in Aleppo, he said: "The regime is testing the rebels' defences in order to uncover their hiding places before annihilating them in a major surgical operation."
Meanwhile, a top UN official said on Thursday that the "main battle" for Aleppo is about to start.
Herve Ladsous, UN peacekeeping chief, said after briefing the UN Security Council on the Syrian conflict that UN observers had not yet seen opposition rebels using the tanks and other heavy weapons that they now have.
"The spiral of violence is still increasing," Ladsous told reporters.
"The focus two weeks ago was on Damascus. The focus is now on Aleppo where there has been a considerable build-up of military means and where we have reason to believe that the main battle is about to start," he added.
Syrian rebels used tanks for the first time to attack a military airport northwest of Aleppo on Thursday, a rebel commander said. Abdel Aziz Salameh told the AFP news agency that his forces had captured four tanks from government forces.
Ladsous said unarmed military observers who had been near Aleppo have seen the heavy weapons.
"We have not yet seen the opposition in the action of using those heavy weapons against government forces. But we know that they have tanks, that they have armoured personnel carriers etcetera - that's a fact," he said.
In a separate development, mobile phone and internet services, cut since Wednesday night, were being gradually restored in Aleppo by Thursday afternoon.
As clashes between government forces and rebels took place nationwide, at least 67 people, including 36 civilians, 16 soldiers and 15 rebels, were killed across the country on Thursday, according to the Syrian Observatory for Human Rights.
In the capital Damascus, heavy fighting erupted in the southern Tadamon district, the Observatory said.
"Syrian troops withdrew from the neighbourhood following heavy clashes with rebel battalions, which resulted in the killing of four rebels and at least three soldiers," the UK-based activist group added.
Also, for the first time, regime forces raided the exclusive Muhajireen neighbourhood of north Damascus, reportedly arresting about 20 young men.