The bodies of 47 women and children, some with their throats slit, have been found in the Syrian city of Homs after a "massacre" that sent families fleeing the area, activists say.
Syria's information minister accused "terrorist gangs" of carrying out the killings in order to incite international pressure on President Bashar al-Assad's regime.
At a meeting of top diplomats of UN Security Council member states in New York, Western foreign ministers demanded that Syrian allies China and Russia stop blocking UN action to halt the bloodshed.
Britain's William Hague said most of the world believed the council had failed in its responsibilities to the Syrian people, while France's Alain Juppe appealed to China and Russia to heed the conscience of the world.
Russia's Sergei Lavrov retorted that change in the Arab world "must not be achieved by misleading the international community or manipulating the Security Council".
The grisly murders in Homs came after UN-Arab League peace envoy Kofi Annan warned on Sunday that the situation in Syria had reached a "dangerous" level, as he failed to conclude a deal to end violence rocking the country during a weekend mission to Damascus.
Hadi Abdallah, a Syrian activist in Homs, told AFP the bodies of 26 children and 21 women, some with their throats slit and others bearing stab wounds, were found after a "massacre" in the Karm el-Zaytoun and Al-Adawiyeh neighbourhoods of the besieged central city.
"Some of the children had been hit with blunt objects on their heads, one little girl was mutilated and some women were raped before being killed," he said.
Activists posted videos online that showed graphic images of charred bodies and children with mutilated and bloodied faces.
Syrian state television also aired gruesome footage showing homes with white walls splattered with blood, bodies of women and children piled on top of each other, and several men, with bullet wounds to the head, lying facing down in a disused building, their hands tied behind their backs.
Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, said news of the killings in Homs had prompted hundreds of families to flee the city for fear of a new massacre.
Syrian state TV said the weekend killings were a clear ploy by "armed terrorist gangs" to grab the spotlight ahead of a meeting later today of foreign ministers from key UN Security Council nations in New York.
"We are used to them committing more crimes before meetings of the UN Security Council," it said, while denouncing "hysteria" in the media over events in Syria.
Information Minister Adnan Mahmoud told AFP: "Terrorist gangs carried out the most horrible massacre in the Karm el-Zaytoun neighbourhood of Homs ... in order to incite international reaction against Syria."
According to the Britain-based Syrian Observatory, more than 8500 people have been killed in Syria since the outbreak of the revolt against Assad's regime a year ago.
Activists said the army launched a new assault on Monday in the restive northwestern province of Idlib and the city of the same name, where they reported residents suffering "indescribable" humanitarian conditions.
"The army bombarded the Dbeit district in Idlib city and pounded the Ath-Thawra neighbourhood, where several buildings collapsed," an anti-regime activist in the city said via Skype, giving his name only as Yasser.
"The humanitarian situation is indescribable, the residents are totally without water and electricity, and communications have been cut off," he added.
While the army controls some parts of the city that were wrested from the rebels in an assault on Saturday, others remain in the hands of the insurgents.