"Most of the victims were shot at close range and the executions were carried out by pro-regime militias," Rupert Colville, the spokesman for the UN High Commissioner for Human Rights, was quoted by the BBC as saying.
The UN statement came as UN-Arab League envoy Kofi Annan was meeting Syrian President Bashar Assad in Damascus.
Colville told reporters here that initial investigations suggested that fewer than 20 of the victims in the village of Taldou, near Houla, were killed by artillery or tank fire.
"Most of the rest of the victims in Taldou," he said, "were summarily executed in two separate incidents.
At this point it looks like entire families were shot in their houses."
The UN spokesman said the conclusions were based on accounts gathered by the UN monitors and corroborated by other sources.
Colville said witnesses blamed pro-government thugs known as shabiha, who came from nearby Alawite villages, for the attacks.
The massacres in villages in Houla, near the central Syrian city of Homs, last week have evoked global ire and western nations like UK, France, US and Australia have begun moves to raise diplomatic pressure on the Assad government.
Activists have posted amateur videos online showing shells exploding in the villages, dismembered bodies strewn on the streets and rows of dead laid out before mass burial.
The UN had earlier said that among the victims were 49 children and 34 women, with some of them having tell-tale signs of bullet holes in their heads.