Western nations are pressing for a response to the massacre in the Syrian town of Houla, with the US calling for an end to President Bashar al-Assad's "rule by murder".
UK Foreign Secretary William Hague has called for an emergency meeting of the UN Security Council this week.
The UN has confirmed the deaths of at least 90 people in Houla, including 32 children under the age of 10.
Syria's foreign ministry denied army involvement, blaming "terrorists".
Spokesman Jihad Makdissi said there was a clash in the area on Friday afternoon between security forces and armed terrorists.
He said hundreds of armed men then gathered, armed with heavy weapons including mortars and anti-tank weapons, and attacked Houla.
The government had sent a team to investigate the killings, he added.
However, activists say the Syrian military bombarded Houla after demonstrations.
Activists say some of the victims died by shelling, while others were summarily executed by the regime militia known as the "shabiha".
Time 'running out'
The killings have sparked a chorus of international condemnation.
Mr Hague said he would lobby Russia, the regime's only major foreign ally, during a visit to Moscow starting on Sunday.
He wrote on his Twitter feed : "Will call on Russia to support rapid and unequivocal pressure on Assad regime and accountability for crimes."
"Time is running out for the Assad regime to adopt the [Kofi Annan peace] plan, implement that plan and stop the torture, abuse and murder of their own people," he told the BBC.
US Secretary of State Hillary Clinton called it an "appalling" massacre, saying President Bashar al-Assad's "rule by murder and fear must come to an end".
In a statement on Saturday, Mrs Clinton called the deaths an "atrocity" and said Washington would increase pressure on "Assad and his cronies" to give up power.
France's Foreign Minister Laurent Fabius said he was making immediate arrangements for a Paris meeting of the Friends of Syria group.
The group includes Western and Arab nations, but not Russia or China, which have blocked previous attempts to introduce UN sanctions.
In a joint statement, UN Secretary-General Ban Ki-moon and UN-Arab League envoy Kofi Annan said the attack was a "flagrant violation of international law".
They said the crime involved "indiscriminate and disproportionate use of force" and violated commitments by Syria's government.
In April, Damascus pledged to implement a six-point plan brokered by Mr Annan, including a ceasefire and the withdrawal of heavy weaponry from urban areas.
Mr Annan is expected to visit Damascus on Monday to try to breathe life into the political process.
But the opposition Free Syrian Army (FSA) said it could no longer commit to the ceasefire unless the Security Council could ensure that civilians were protected.
In a statement, the FSA said that if urgent steps were not taken, then Mr Annan's plan was "going to hell".
It said killings in Syria were taking place "under the eyes of the UN observers," and called on states to "announce the failure of the Annan plan".
Violence in Syria has continued despite the deployment of some 260 UN observers sent to oversee a ceasefire.
The head of the observer mission in Syria, Maj Gen Robert Mood, called the massacre a "brutal tragedy", but did not say who carried out the murders.
After a UN visit to the town on Saturday, he said they could confirm "the use of small arms, machine gun, artillery and tanks".
"Whoever started, whoever responded and whoever carried out this deplorable act of violence should be held responsible," Gen Mood said.
The UN has confirmed the deaths of at least 90 people, but the opposition Syrian National Council, as well as rights groups, put the toll higher.
Activists have complained that they called for help as the massacre was taking place.
Abu Emad, speaking from Houla, said their appeals to UN monitors failed to produce action.
"We told them at night, we called seven of them. We told them the massacre is being committed right now at Houla by the mercenaries of this regime and they just refused to come and stop the massacre," he said.