Activists and opposition groups say Syrian troops and rebels have clashed for a second day in the northern city of Aleppo.

They say the overnight clashes with heavy machine guns were some of the fiercest to date in the heart of Syria's northern commercial hub.

Aleppo has been largely shielded from the violence that has plagued other Syrian cities over the course of the uprising against President Bashar Assad, now in its 17th month.

Aleppo-based activist Mohammad Saeed said dozens of rebels from the Free Syrian Army were now in the city. He said fighting was mostly in the Salaheddine district in the city center.

The Local Coordination Committees activist network and the Britain-based Syrian Observatory for Human Rights said Friday's fighting forced many residents to flee to safer areas.