The United States has condemned Syria for the deaths of dozens of children in an apparent artillery barrage, and said "rule by murder" must end.
Secretary of State Hillary Clinton described the incident as an "atrocity" and said Washington would work with its international allies to increase the pressure on President Bashar Assad and his "cronies".
"Rule by murder and fear must come to an end," she declared.
Her remarks come as the head of the UN mission there warned of "civil war" after his observers counted more than 92 bodies, 32 of them children, in the central town of Houla following reports of a massacre there.
UN Secretary General Ban Ki-moon joined a chorus of international condemnation amid mounting calls for world action to halt the bloodshed.
The rebel Free Syrian Army (FSA) said it was no longer committed to the UN-backed peace plan for Syria unless there was prompt UN intervention to protect civilians, and called for air strikes against regime forces.
UN head of mission Major General Robert Mood called what happened in Houla a "brutal tragedy".
Gen Mood told Sky News : "Whichever way you look at this, whoever started and whoever responded and whoever contributed, to this deplorable act of violence should be held responsible.
"This indiscriminate and disproportionate use of force is unacceptable and I would also say unforgivable."
A human rights monitoring group said 114 people had been killed in Houla.
Mr Ban and Kofi Annan, the UN and Arab League envoy who brokered a truce which has been flouted daily since it came into effect April 12, said the attack was a "brutal" breach of international law.
"This appalling and brutal crime, involving indiscriminate and disproportionate use of force, is a flagrant violation of international law," a UN spokesman quoted the pair as saying.
British Foreign Secretary William Hague said "we will be calling for an urgent session of the UN Security Council in the coming days".
Amateur videos posted on YouTube showed horrifying images of dead children, with at least one child's head partly blown away.
The massacre will heighten the importance of a trip Mr Annan is to make to Syria "soon", his spokesman said, while the Security Council is due to discuss Syria on Wednesday.
UAE Foreign Minister Sheikh Abdullah bin Zayed al-Nahayan called for an urgent Arab League meeting, saying the "massacre shows the failure of Arab and international efforts to stop the violence against civilians in Syria".
The six-nation Gulf Cooperation Council, of which the UAE is a member, urged the international community to "assume its responsibilities" to end the bloodshed.