Syrian troops and armoured vehicles have pushed into a rebel-held district of Aleppo and struck back in the capital, Damascus, against fighters emboldened by a bomb attack that killed top officials of President Bashar al-Assad's government.
Opposition activists in Aleppo, Syria's biggest city and a northern commercial hub, said that hundreds of families were fleeing residential areas on Saturday after the military swept into the Salaheddine 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, who declined to be named, said by phone from the city.
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The Syrian Observatory for Human Rights said the fighting started spreading on Saturday afternoon.
"Violent clashes are taking place between Syrian regime forces and rebel fighters in the Sakhour neighbourhood and the Haydariya area," the UK-based group said.
The Observatory also reported heavy clashes at the entrance of al-Rastan, a rebel stronghold in Homs province, saying troops were trying to enter the town.
There were also reports of renewed clashes in two districts of the Syrian capital.
In the central city of Homs, at least two people were killed in a prison mutiny, opposition activists said.
They said some guards had joined in the mutiny, which broke out shortly after midnight, but that government forces from a nearby intelligence base had arrived to crush the rebellion.
Some activists said that two prisoners were killed, while others put the death toll at four and said the mutiny was continuing, although the prison was surrounded.
Meanwhile, UN chief Ban Ki-moon said on Saturday that an envoy will be dispatched to Syria to assess the situation as violence increases in the embattled country, adding that Assad's government must stop "the killing and the use of heavy weapons" against population centres.
Multiple control points
Rebels have also managed to seize border checkpoints in recent days, most significantly, taking control of a key border crossing with Iraq.
Al Jazeera's Jane Arraf, reporting from Erbil, Iraq, noted that this crossing, the second at the Syrian-Iraqi border, is important because it might signal a sea change among a group ordinarily loyal to Assad.
"Significantly, this is the crossing from the Kurdish regions of Syria. Now, those Kurdish regions have traditionally stayed out of this fight - they've been relatively calm," said Arraf.
"This one could mark a new turn in the conflict," she said.
Fighters have held the Abu Kamal crossing between Syria and Iraq since Thursday, and despite government forces having heavily shelled it on Friday night, it remained in rebel hands on Saturday.
In addition, the Free Syrian Army (FSA) has taken control of the Bab al-Hawa crossing on the border with Turkey.
Al Jazeera's Andrew Simmons, reporting from Cilvegozu on the Turkish side of the frontier, said the FSA had established a proper checkpoint, controlling any traffic coming through.
"The Free Syrian Army is in control of the first part of the main road into Syria but the regular Syrian army has some sort of base a few kilometres down the road," he said. "We spoke to Syrian who've been fleeing. They say they passed through two separate control points, one manned by the regular army and one by the FSA."
Simmons said the border was not officially closed, but officials were "discouraging everyone" from crossing. He added that Turks were being turned away from the border, although some Syrian refugees had made it across.
South of Syria, our correspondent Stefanie Dekker, reporting from Ramtha, Jordan, said that the Free Syrian Army also attempted to take control of the Nasib border crossing, but were repelled back by government forces.
Opposition 'pushed out' of Damascus
Meanwhile, a tense calm prevailed in the capital, Damascus, after six days of intense fighting.
The army deployed tanks on Friday to push rebels out of al-Midan neighbourhood, where fighting left cars gutted and fighters' bodies in the streets.
Al Jazeer'a Rula Amin, reporting from Beirut, Lebanon, said that the fighters seemed to have been "pushed out" of the capital.
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"For example, in al-Midan, they are no longer in control, and the neighbourhood is no longer off limits to the government forces," said Amin.
"What we are seeing is the government pushing in each neighbourhood it thinks the rebels are [present]."
Battles involving troops bringing in tanks, helicopters and mortars have turned parts of Damascus into combat zones, and sent thousands of Syrians streaming across the border into neighbouring Lebanon.
More than 200 people were killed across the country in a single day on Friday, activists said.
Despite being forced out of al-Midan, the armed opposition appear to have gained momentum across the country in recent days.
Four senior officials were killed after bomb went off during a high-level security meeting in Damascus on Wednesday, according to government media.