Beirut - Syria on Thursday blamed up to 800 rebel fighters for the massacre in central Syria last week that killed more than 100 people, nearly half of them children, in its most comprehensive explanation to date of the bloodshed.
The narrative starkly contradicted accounts of witnesses who blamed "shabiha" or the shadowy gunmen who operate on behalf of President Bashar al-Assad's regime.
The UN also said it had strong suspicions those pro-regime gunmen were responsible for much of the carnage on Friday in a cluster of villages known as Houla.
Facing international outrage over the killings, Damascus launched its own investigation into the deaths and announced that special prayers for the victims would be held at mosques across the country on Friday.
The UN chief warned of civil war and pleaded with the regime to stop its attacks.
At a news conference on Thursday, Qassem Jamal Suleiman, who headed the government's investigation into the massacre, categorically denied any regime role. He said hundreds of rebel gunmen carried out the slaughter after launching a co-ordinated attack on five security checkpoints.
The aim, he said, was to frame 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, adding that the victims were families who refused to oppose the government or carry arms.
Foreign Ministry spokesperson Jihad Makdissi said the aim was to create sedition in Syria.
"There are people in dark rooms working night and day to target Syria ... and the way to do it is to ignite civil strife," he told reporters at the same news conference. "They will not succeed."
The Houla massacre was one of the deadliest incidents since the uprising against Assad's hardline regime started in March last year. Activists say about 13 000 have been killed in 15 months.
The area is still under attack.
The government focused its shelling on Thursday on the Houla village of al-Tibeh.
The activist group Syrian Observatory for Human Rights reported that some residents fled to nearby towns and villages "fearing a new massacre" as the area again came under fire.
Persistent bloodshed despite a cease-fire agreement has raised pressure on the international community to act.
But Secretary of State Hillary Clinton laid out the clearest case yet for why the Obama administration is reluctant to intervene militarily in Syria even as the US expressed revulsion over the Houla killings.
Clinton said Russia and China would have to agree before the US and other nations engage in what could become a protracted conflict in support of a disorganised rebel force.
"We're nowhere near putting together any type of coalition other than to alleviate the suffering," Clinton told reporters on Thursday after meeting with top officials in Denmark, a key contributor to last year's Nato-led mission against Muammar Gaddafi in Libya.
Russia's continued support for Assad "is going to help contribute to a civil war", Clinton warned.
UN Secretary General Ban Ki-moon called on Syria to stop the bloodshed and also warned of a protracted conflict.
Nearly 300 UN observers have been deployed around Syria to monitor a cease-fire that was supposed to go into effect on 12 April as part of a peace plan negotiated by international envoy Kofi Annan.
But the plan has unravelled amid daily visit and the images from the Houla massacre caused outrage to spike.
"The massacre of civilians of the sort seen last weekend could plunge Syria into a catastrophic civil war - a civil war from which the country would never recover," Ban said in Istanbul.
He added that the international community was united in demands that the Syrian government act on its responsibilities to its people.
"Let me state plainly, however: The UN did not deploy in Syria just to bear witness to the slaughter of innocents," he added. "We are not there to play the role of passive observer to unspeakable atrocities."
Annan, meanwhile, arrived in Lebanon, where 10 people were killed in clashes between pro- and anti-Syrian groups earlier this month, raising fears the conflict could spill over to neighbouring countries.
In the wake of last week's massacre, the United States, Western and Asian nations expelled Syrian diplomats in protest.