Heavy fighting reported in Aleppo

Syrian troops have clashed with rebels in the city of Aleppo for a second day, forcing inhabitants to flee to safer areas in some of the fiercest fighting to date in a key bastion of support for President Bashar Assad, activists said.

The rebels are trying to seize the momentum after a week of battles in the capital Damascus, including a bombing that struck at the heart of the regime, killing four high-level officials.

Two days of clashes in Aleppo's Salaheddine district brought sustained fighting to the city's centre for the first time since the uprising began in March 2011.

The city, a commercial hub and Syria's largest population centre, has remained largely loyal to Assad and been spared the kind of daily bloodshed that has plagued other cities.

But Aleppo-based activist Mohammad Saeed said dozens of fighters from the rag-tag Free Syrian Army entered Aleppo from the countryside and were fighting regime troops from inside.

"This night was very bad, there were huge explosions and the gunfire didn't stop for several hours," he said via Skype. "The uprising has finally reached Aleppo."

Damascus and Aleppo are both home to elites who have benefited from close ties to the Assad regime, as well as merchant classes and minority groups who fear their status will suffer if the president falls.

But for months, rebels have been gaining strength in poorer towns and cities in the Aleppo countryside, gaining footholds near the Turkish border.

Activists and residents reported a tense calm in Damascus on Saturday, although sporadic gunfire and explosions could be heard.

Two residents who did not want to be identified for safety reasons said by telephone that the fighting peaked between 1am and 3am local time. One of the residents said most shops in the capital were closed on Saturday and traffic was light.