A Syrian investigation into the Houla massacre has blamed the atrocities on rebels trying to provoke international intervention.
The official in charge of the inquiry categorically denied any government role in last week's killings.
Activists say Syrian troops or pro-government militia were behind the deaths of more than 100 people - many of them children.
The US ambassador to the UN called the Syrian account "a blatant lie".
"There is no factual evidence, including that provided by the UN observers that would substantiate that rendition of events," Susan Rice said.
UN observers have said government forces were active in the area at the time.
General Qassem Jamal Suleiman, who headed the Syrian government's commission of inquiry into the massacre, said hundreds of rebel gunmen carried out the slaughter after launching a co-ordinated attack on five security checkpoints.
He told a news conference that the aim had been to implicate the government and to ignite sectarian strife in Syria.
"Government forces did not enter the area where the massacre occurred, not before the massacre and not after it," he said.
He said the victims had been families "who refused to oppose the government and were at odds with the armed groups".
"The aim of these armed groups is to bring foreign military intervention against the country in any form and way," he added.
The killings in Houla triggered worldwide condemnation and led many Western powers to expel Syrian diplomats.
UN observers said some of the victims were killed by shell fire but most appeared to have been shot or stabbed at close range. The dead included 49 children and 34 women.
The UN Human Rights Council - the world's leading human rights body - is due to hold an emergency meeting on Syria on Friday at which it is expected to condemn the violence in Houla.
Syria has said special prayers for the victims will be held at mosques across the country on Friday.
A draft resolution, backed by the EU, condemns what it calls "the wanton killings of civilians... by pro-regime elements", and demands that Syria allow human rights investigators and aid agencies into the country immediately.
About 300 UN observers are currently in Syria as part of a six-point peace plan brokered by UN-Arab League envoy Kofi Annan.
A key element of the plan is a ceasefire supposed to have gone into effect on 12 April. However, reports of violence and deaths have since continued daily.
The BBC's Paul Wood, who was recently in Syria, says the ceasefire exists in name only.
On Thursday, US Secretary of State Hillary Clinton criticised Russia's policy toward its ally Syria, saying it was contributing towards a potential civil war.
Russia and China have both renewed their opposition to tougher UN Security Council action against Syria.
Mrs Clinton said the case for military intervention was growing stronger every day.
"[The Russians] are telling me they don't want to see a civil war. I have been telling them their policy is going to help to contribute to a civil war," she told an audience in Copenhagen.
On Friday, Ms Rice also criticised Moscow over reports from human rights groups and Western diplomats that a Russian ship had recently delivered weapons to Syria.
Activists say as many as 15,000 people have been killed since the revolt against President Bashar al-Assad began in March 2011, although the government disputes the figures.