Syrian soldiers and armoured troops have pushed into a rebel-held district of Aleppo after striking back in Damascus against fighters emboldened by a bomb attack against President Bashar al-Assad's inner circle.
Activists in Aleppo, Syria's biggest city and a northern commercial hub, said hundreds of families were fleeing residential districts after the military swept into the Saladin district, which had been in rebel hands for two days.
Fighting was also reported in the densely-populated, poor neighbourhood of al-Sakhour.
"The sound of bombardment has been non-stop since last night. For the first time we feel Aleppo has turned into a battle zone," a housewife said by phone from the city.
An escalation in the fighting in Aleppo would prove another challenge to Assad, still reeling from the assassination of four of his top security officials and a six-day attack on the capital which rebels have named "Damascus Volcano".
The president has not spoken in public since the killings, and failed to attend funeral ceremonies for his brother-in-law and two other slain officials on Friday.
The clashes in Aleppo came as UN Secretary General Ban Ki-moon said he was sending his peacekeeping chief Herve Ladsous and top military adviser Gen. Babacar Gaye to Syria to assess the situation.
In Damascus, Assad's forces hit back overnight. Helicopters and tanks aimed rockets, machineguns and mortars at pockets of lightly armed rebel fighters who moved through the streets on foot, attacking security installations and roadblocks.
Residents who toured the city on Saturday said it was relatively quiet, though gunfire and explosions could still be heard intermittently in some areas.
Most shops were closed and there was only light traffic - although more than in recent days. Some police checkpoints, which had been abandoned earlier in the week, were manned again.
Most petrol stations were closed, having run out of fuel, and the few that were open had huge lines of cars waiting to fill up. Residents also reported long queues at bakeries and said vegetable prices had doubled.
"I feel depressed and lonely because I have to stay indoors as there is nothing good outside. Everyone else is depressed as well," said a woman in her 50s in west Damascus who supports Assad's opponents.
An opposition activist said he had sneaked back into the Midan district, which Assad's forces seized back from rebel control on Friday, only to find his house looted.
"The doors were broken and I walked into several houses which were in the same condition," said Fadi al-Wahed. "Safes were broken into, drawers broken and furniture and television screens missing. Three army trucks were parked under the ring road flyover with loot."