Syria crisis: Heavy clashes in second city of Aleppo
There have been heavy clashes between security forces and rebels in Syria's second city of Aleppo, activists say.
The fighting was centred on the Salah al-Din district, but had also spread to Sakhur and Haydariya, the Syrian Observatory for Human Rights reported.
Aleppo has so far been spared the daily bloodshed seen in other cities since the uprising began in March 2011.
The violence came a day after the UN Security Council voted to extend the UN observer mission for a "final" 30 days.
A resolution stated that after that period the monitors would leave if they were unable to carry out their job of verifying the peace plan brokered by the UN and Arab League's special envoy, Kofi Annan.
Their mandate may be renewed if the use of heavy weapons ends.
The observers' work has been mostly suspended since June because of the escalating violence, which reportedly left more than 300 people dead on Thursday and another 200 on Friday.
On Saturday, UN Secretary General Ban Ki-moon urged the Security Council to forge a united way forward and exercise its collective responsibility.
Mr Ban also said he would send his Under-Secretary General for Peacekeeping Operations, Herve Ladsous, to Syria and had asked his top military adviser to take charge of the observer mission.
Activists said the clashes in Aleppo between troops and members of the rebel Free Syrian Army (FSA) began in Salah al-Din on Friday morning. By Friday afternoon, they had spread into Sakhur and Haydariya.
Aleppo-based activist Mohammed Saeed said the fighting continued throughout the night until Saturday morning, most of it in Salah al-Din.
"Last night was very bad," he told the Associated Press. "There were huge explosions and the gunfire didn't stop for several hours."
"The uprising has finally reached Aleppo," he added.
Mr Saeed said dozens of FSA fighters had entered from the countryside. Aleppo is not far from Turkey, where the FSA commanders are based.
The Local Co-ordination Committees, an activist network, said there had been an "exodus" of Salah al-Din residents "because of fear of a regime bombardment and offensive".
The BBC's Jim Muir in Beirut says fighting in Aleppo is ominous for President Bashar al-Assad's government, since the city has so far stayed out of the uprising.
So too had the capital, Damascus, but those days are now over, our correspondent adds.
The capital saw heavy clashes again on Saturday as government troops hit back at rebels who instigated a number of attacks last week, including killing four high-ranking officials with a bomb.
There were reports of heavy shelling and of helicopters firing rockets into the Sayida Zeinab district.
There was also a violent and prolonged battle in the rebel-held town of Talbasiya, north of the city of Homs in central Syria. Activists said government forces tried to storm the town then bombarded it heavily.
The Syrian Observatory for Human Rights said at least 130 people were killed on Saturday across the country, including 64 civilians.