UNITED NATIONS - The United Nations led calls Saturday for urgent international action on Syria after reports of a horrific massacre by regime forces that killed 92 people, more than a third of them children.

UN Secretary General Ban Ki-moon and UN-Arab League envoy Kofi Annan jointly condemned the "appalling and brutal crime," which involved "indiscriminate and disproportionate use of force," and was "a flagrant violation" of international law and commitments by Syria's government not to use heavy weapons or violence.

"Those responsible for perpetrating this crime must be held to account," the UN chief and Annan added.

The UN mission said 92 bodies, 32 of them children aged less than 10, had been counted in the central Syrian town of Houla after reports of an artillery bombardment by President Bashar al-Assad's forces.

The offensive resulted in one of the bloodiest episodes since his regime launched a brutal crackdown on opponents in March last year that has left thousands dead.

Britain said it was in urgent talks with allied countries on "a strong international response" while France said it was making plans to host a "Friends of Syria" meeting in the wake of the latest deadly violence.

The UN mission chief in Syria, Major General Robert Mood, condemned what he described as a "brutal tragedy" in Houla, where he said the bodies of victims had been counted.

"Those using violence for their own agendas will create more instability, more unpredictability and may lead the country to civil war," he told reporters in Damascus, describing the violence as "indiscriminate and disproportionate."

Amateur videos posted on YouTube, apparently from Houla in Homs province, showed horrifying images of children lying dead on a floor. Some were badly mangled, with at least one child's head partly blown away.

"We are consulting urgently with our allies on a strong international response, including at the UN Security Council, the EU and UN human rights bodies," British Foreign Secretary William Hague said in a statement.

London would seek an urgent session of the Security Council in coming days, in response to "credible and horrific reports that a large number of civilians have been massacred" by Syrian forces in Houla.

"Our urgent priority is to establish a full account of this appalling crime and to move swiftly to ensure that those responsible are identified and held to account," Hague said.

The Syrian regime must cease all military operations in accordance with a peace plan brokered by UN-Arab League envoy Kofi Annan, and to allow monitors "full and immediate access" to Houla and other conflict areas, Hague added.

The rebel Free Syrian Army said after the killings that it was no longer committed to the UN-backed peace plan for Syria unless there was prompt UN intervention to protect civilians.

The plan drawn up by Annan technically began on April 12, but violence and bloodshed have not stopped and a ceasefire that formed part of the plan has been breached daily.

France's new Foreign Minister Laurent Fabius said he was making immediate arrangements for the Friends of Syria nations -- which include Britain but not key UN players China and Russia -- to meet in Paris.

"I condemn the atrocities committed daily by Bashar al-Assad on his own people," Fabius said. "With these new crimes his murderous regime plunges Syria further into horror and threatens regional stability."

Fabius said he would speak to Annan on Sunday, adding: "In the face of horror, the international community must mobilize still further to stop the martyrdom of the Syrian people."

German Foreign Minister Guido Westerwelle said he was "shocked and horrified" at the killings.

"It is appalling that the Syrian regime does not put an end to the brutal violence against its own people," Westerwelle said in a statement. "Those responsible for this crime must be punished."