Syrian security forces have killed at least 12 people in two days during an ongoing military operation near the Turkish border in the country's northwest as anti-government protests broke out for the first time in the key strategic city of Aleppo.

Rights activists said Thursday that Syrian troops backed by tanks and helicopters consolidated their hold on several villages in Idlib Province.

Pro-government forces also appeared to prevent residents from crossing into Turkey, where more than 12,000 Syrians have fled to escape the violence in their homeland. Hundreds more Syrian refugees have left for Lebanon.

Turkish officials said only five Syrians made it across the border Thursday, the lowest number in days.

Elsewhere, in Syria's second largest city, Aleppo, video footage posted to the Internet showed pro-government gangs attacking hundreds of protesters in at least two locations Thursday.

Plainclothes police later dispersed the crowds, and allowed government supporters to hold a counter-demonstration.

A Facebook group of pro-democracy activists had called for Thursday's anti-government rallies in Aleppo, a key commercial hub populated by much of Syria's small merchant class, many of whom support President Bashar al-Assad.

The group also urged Syrians to rally nationwide Friday and call for Mr. Assad's ouster.

Details of events in Syria are difficult to independently confirm because the government allows very few foreign news reporters into the country and restricts their movements.

Rights groups say more than 1,400 people have been killed in the violence, most of them unarmed protesters.