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 war monitoring group said more than 1,000 were killed in revenge attacks and clashes between Syrian security forces and loyalists of ousted President Bashar Assad.

Entire families including women and children were killed in Syria's coastal region as part of a series of sectarian killings by rival groups during an insurgency by Bashar al-Assad loyalists, the U.N human rights office said on Tuesday.

Armed groups killed entire families, including women and children, during an outbreak of sectarian violence in Syria last week, the United Nations human rights office said on Tuesday.