A senior Syrian military officer was assassinated over his unwillingness to open fire at anti-government protesters, websites linked to Syrian opposition groups said on Monday.

According to the report, a Syria colonel was killed along with two of his sons as part of what the opposition websites are calling a new policy by Bashar Assad' regime, geared at getting rid of officers who show sympathy toward protesters.

The report came as human rights groups claimed earlier Monday that Syrian forces killed 12 protesters overnight in the central city of Homs in clashes after the death of a tribal leader in custody.

"Homs is boiling. Security forces and the regime thugs have been provoking armed tribes for a month now. But civilians in large numbers also took to the streets in different areas of Homs last night and they were shot at in cold blood," the rights campaigner told Reuters.

The violence comes shortly after Syrian security forces killed three mourners on a highway outside the town of Talbiseh, slightly north of Homs, on Sunday when they opened fire on a funeral that had turned into a demonstration, two witnesses said.

They had been attending the funeral of a man killed the day before by security forces, the witnesses said. The mourners marched across a bridge on the highway near where security police and gunmen loyal to President Bashar Assad were stationed.

Despite Assad's promises of reform such as lifting the decades-old emergency laws, protests continued to erupt in several cities in Syria with thousands of people taking to the streets to call for greater freedom.

Thousands of protesters in the southern town of Suweida participated in a rally to mark Evacuation Day, commemorating the departure of the last French soldiers 65 years ago and Syria's proclamation of independence.