There have been heavy clashes between security forces and rebels in Syria's second city of Aleppo, activists say, according to BBC News.

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.

Activists said the clashes in Aleppo between troops and members of the rebel Free Syrian Army (FSA) began in Salah al-Din on Friday July 20 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".