UN observers rushed on Saturday to a town in central Syria where scores of civilians were reportedly massacred, including 32 children, as the armed opposition renewed calls for air strikes on regime forces.
The Free Syrian Army (FSA) also announced it was no longer committed to the UN-backed peace plan for Syria unless there was prompt UN intervention to protect civilians.
The shelling of the town of Houla by regime forces began at around midday on Friday and continued until dawn on Saturday, the Syrian Observatory for Human Rights said, adding that UN observers had visited the area.
In Damascus, UN mission chief General Robert Mood said in a statement the monitors had counted more than 92 bodies in Houla, and called the incident a "brutal tragedy."
"This morning UN military and civilian observers went to Houla and counted more than 32 children and over 60 adults killed," the statement said.
French Foreign Minister Laurent Fabius condemned the massacre and said he was "making immediate arrangements for a Friends of Syria group meeting in Paris."
Earlier the rebel (FSA) urged the Friends of Syria nations to launch air strikes against President Bashar al-Assad's forces.
Turkey-based General Mustafa Ahmed al-Sheikh, head of the FSA's military council, urged "an appropriate stance after the heinous crime committed by Assad's assassin regime in the Houla region."
"We are calling urgently on the Friends of Syria to create a military alliance, outside of the UN Security Council, to carry out targeted strikes against Assad's gangs and the symbols of his regime," he said.
Later the FSA issued a statement saying: "We announce that unless the UN Security Council takes urgent steps for the protection of civilians, (Kofi) Annan's plan is going to go to hell."
The peace plan drawn up by UN-Arab League envoy Kofi Annan technically began on April 12, and days later UN observers began deploying in Syria to monitor the truce. But the violence and bloodshed have not stopped.
Amateur videos posted on YouTube showed horrifying images of children lying dead on a floor. Some were badly mangled, with at least one child's head partly blown away.
The Britain-based Observatory accused the international community of being "complicit" in the killing and standing "silent in the face of the massacres committed by the Syrian regime."
Earlier, opposition Syrian National Council spokeswoman Basma Kodmani said "more than 110 people were killed (half of whom are children) by the Syrian regime's forces" in Houla.
"Some of the victims were hit by heavy artillery while others, entire families, were massacred."
"The Syrian National Council urges the UN Security Council to call for an emergency meeting to examine the situation in Houla and to determine the responsibility of the United Nations in face of such mass killings, expulsions and forced migration from entire neighbourhoods," she added.
State news agency SANA blamed "armed terrorist groups" for the killings, adding that "clashes led to the killing of several terrorists and the martyrdom of several members of the special forces."
Regime forces pounded rebel positions elsewhere in Homs province on Saturday, according to the Observatory, which reported 18 people killed in violence across the country, including 13 civilians.
Saturday's dead included a civilian killed in the Homs rebel bastion of Qusayr, the Observatory said, adding that three others died elsewhere in the restive province.
Protests took place on Saturday against the reported massacre in Houla, anti-regime activists said.
In Kfarnabel, in the northwestern province of Idlib, scores of men and women took to the streets to honour the dead, chanting: "We sacrifice our soul and life for you, O people of Houla," according to an amateur video on YouTube.
Protesters were also angry with the international community.
One demonstrator held up a sign reading: "Annan is singlehandedly responsible for the Houla massacre," blaming the former UN chief for continued violence.
Annan's six-point peace plan included a ceasefire that has been breached daily since going into effect on April 12.
The envoy is to travel to Syria "soon" as he continues efforts to find a peaceful solution to the crisis, his spokesman Ahmad Fawzi said. Diplomats in Geneva said Annan would visit Damascus early next week.
France's foreign minister condemned "the atrocities committed daily by Bashar al-Assad on his own people. With these new crimes his murderous regime plunges Syria further into horror and threatens regional stability."
Fabius said UN observers should complete their deployment in line with Annan's plan.
Saying he would speak to Annan on Sunday, Fabius added: "In the face of horror, the international community must mobilise still further to stop the martyrdom of the Syrian people."
More than 12,600 people have been killed in Syria in the revolt against Assad's rule, including nearly 1,500 since the UN-backed came into effect, according to Observatory figures.