Most of the 108 people killed in Syria's Houla region on Friday were summarily executed, the UN says.
A spokesman for the UN's human rights office says witnesses told investigators that pro-regime militias carried out most of the killings.
Survivors have described gunmen entering homes, firing indiscriminately and slitting the throats of children.
The UN statement came as UN-Arab League envoy Kofi Annan met President Bashar al-Assad in Damascus.
Rupert Colville, spokesman for the UN High Commissioner for Human Rights, told journalists in Geneva 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 added, "were summarily executed in two separate incidents."
He said 49 children and 34 women were among the victims.
Eyewitnesses told the BBC that those who carried out the killings were pro-government shabiha militiamen. Survivors said they had hid or played dead.
UN observers who visited Taldou said many of the victims had been killed by close-range gunfire or knife attacks.
Syrian leaders insist that the massacre was the work of rebels - whom they called "terrorists" - who carried out the killings to derail the peace process and provoke intervention by Western powers.
Mr Annan called the massacre "an appalling moment with profound consequences".
Ahead of his meeting with President Assad on Tuesday, the former UN secretary general said the Syrian government had to take "bold steps" to show it was serious about peace.
The BBC's Jim Muir, in neighbouring Lebanon, says it is make-or-break time for Mr Annan's peace plan, and he has to get something out of his visit to stop the drift towards a vicious sectarian civil war.
Under the plan, both sides were meant to stop fighting on 12 April ahead of the deployment of monitors, and the government was to withdraw tanks and forces from civilian areas.
Western leaders have expressed horror at the killings, and the UK, France and US have all begun moves to raise diplomatic pressure on the Assad government.
The French government said "the murderous folly" of the Damascus regime threatened regional security, and announced it was expelling the Syrian ambassador in Paris.
A meeting of the so-called Friends of Syria group is to meet in France in July, President Francois Hollande's office said on Tuesday.
However Russia, which supplies arms to the Syrian government and has blocked UN resolutions calling for action against Damascus, has blamed both sides for Friday's massacre.
Russian Foreign Minister Sergei Lavrov expressed concern that "certain countries" were beginning to use the Houla massacre "as a pretext for voicing demands relating to the need for military measures to be taken".