BEIRUT (AFP) - At least 15 civilians were "summarily executed" by regime forces in a neighbourhood of the central Syrian city of Homs overnight, the Syrian Observatory for Human Rights said on Wednesday.

"After regime forces raided the neighbourhood of Shammas, 15 civilians were found summarily executed," Rami Abdel Rahman, head of the Britain-based watchdog told AFP, qualifying the killings a "massacre."

The executions came one day after regime forces were accused of committing another massacre in Khan Sheikhoun in northwest Idlib, when they opened fire on a funeral procession and reportedly killed 20 people.

Abdel Rahman said a 43-year-old Muslim cleric who had six children was among those killed in Homs. "Everybody loved this cleric, because he called for national unity," said Abdel Rahman, who added he was involved in charity work.

The killings were reported hours after regime troops shot dead at least five people in a new assault in Khan Sheikhun and opened fire on a refugee camp in southern Syria on Wednesday, the Observatory said.

Four others died of wounds suffered on Tuesday during the Khan Sheikhoun funeral, according to the Britain-based group.

Elsewhere on Wednesday, three civilians were killed as troops fired on a refugee camp in southern Daraa province, the Observatory said.

Regime forces also killed a young man in a raid on Mleiha al-Aatsh village of Daraa province, the watchdog said. And in the province of Homs, a civilian was killed in Rastan under regime bombardment, according to the Observatory.

The watchdog updated its toll of people killed in Syria on Tuesday to 64.

The bloodshed comes despite a truce brokered by UN-Arab League envoy Kofi Annan as part of a six-point plan aimed at ending violence that has swept Syria since March 2011, when the uprising against President Bashar al-Assad erupted.

More than 12,000 people, the majority of them civilians, have died since the Syrian uprising began, according to the Observatory, including more than 900 killed since the truce came into effect.