Thousands protested on Wednesday in several areas of Syria after a video circulated showing an attack on an Alawite shrine in the country's north, a war monitor and witnesses said.

Syria's new Islamist-led authorities said the footage was "old" and that "unknown groups" were behind the attack, saying "republishing" the video served to "stir up strife", a day after hundreds protested in Damascus against the torching of a Christmas tree.

The Syrian Observatory for Human Rights said "thousands of people" took to the streets on Wednesday, with major demonstrations in the coastal cities of Tartus and Latakia, provinces that are the heartland of the Alawite minority which deposed ruler Bashar al-Assad hails from.

The Britain-based Observatory also reported protests in parts of the central city of Homs and other areas including Qardaha, Assad's hometown.

State news agency SANA said police in central Homs imposed a curfew from 6:00 pm until 8:00 am local time on Thursday, while local authorities in Jableh also announced a night-time curfew.

The Observatory said the protests erupted after a video began circulating earlier on Wednesday showing "an attack by fighters" on an important Alawite shrine in the Maysaloon district of Syria's second city Aleppo.

It said five workers were killed, adding that the shrine was set ablaze.

Observatory chief Rami Abdel Rahman said the video was filmed early this month, after rebels led by Islamist group Hayat Tahrir al-Sham (HTS) launched a lightning offensive and seized control of major cities including Aleppo on December 1, ousting Assad a week later. (AFP)