Syrian soldiers and armoured troops pushed into the rebel-held district of Aleppo today 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.
An escalation in the fighting in Aleppo would prove another challenge to Dr 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 yesterday.
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.
"I am sending my undersecretary general for peacekeeping operations (Herv Ladsous) to Syria to assess the situation as well as the top UN military advisor (Gen Babacar Gaye) to lead UNSMIS (mission) in this critical phase," Mr Ban said after meeting Croatian officials in the Adriatic resort of Brijuni.
Mr Ban's remarks came a day after the UN Security Council approved a 30-day extension for an unarmed observer mission and two days after Russia and China vetoed a resolution to impose further sanctions on the government of Dr Assad.
"The unanimous vote yesterday on Resolution 2051 is a constructive sign," Mr Ban said.
He called on the UN Security Council to "redouble efforts to forge a united way forward and exercise its collective responsibility".
"The Syrian government has manifestly failed to protect civilians and the international community has collective responsibility to live up to the UN Charter and act on its principles," he said.
Syrian government forces pounded rebels in Damascus overnight, battling to reverse opposition gains in the aftermath of the assassination of Assad's security chiefs.
Lightly-armed fighters have been moving through the streets on foot and attacking security installations and roadblocks.
But the heart of the city was quiet by this morning residents told Reuters.
"I am deeply distressed by the rising death toll and the growing number of people who have been forced to flee their homes within the country and across the borders," Mr Ban said.
He added that the extension of the mandate of the UN observers was "a strong signal" that responsibility for ending the violence was "above all on the parties" in the conflict.
The UN chief said the Syrian government must stop the use of heavy weapons against the population and that all armed operations must end.
"The path towards peace is clear. There must be an end to killing and human rights violations and a rapid move towards the peaceful Syrian-led political transition and dialogue."
Reuters