Syria's fragile transition from decades of Assad family brutal rule has been greatly shaken with an outburst of sectarian retribution and killings, posing a serious challenge to the new leadership led by Ahmad Sharaa.

A Syrian military operation against remnants of the former Assad regime has now ended, having sparked some of the worst violence seen in the country in years.

A monitoring group says 973 civilians have been killed in revenge attacks against the Alawite sect since Thursday.

A Syrian security source said the pace of fighting had slowed around Latakia, Jabla and Baniyas, while forces searched surrounding mountainous areas where an estimated 5,000 pro-Assad insurgents were hiding.

Hundreds of people were killed in Syria over four days after violent clashes between the army and loyalists of the deposed president Bashar al-Assad swept its western coastal region.