Activists denounced "the massacre" of 47 women and children, whose bodies were found in the flashpoint Syrian city of Homs, as major powers remained divided over what action to take amid a growing clamour for foreign intervention.
At a UN Security Council meeting in New York, Western governments stepped up their pleas to Russia and China to end their blockage of action over the Syrian government's deadly assault on protest cities such as Homs.
But Russia showed little sign that it would change its stance, with Foreign Minister Sergei Lavrov slamming "risky recipes" which he said risked increasing conflict in the Middle East.
International peace envoy Kofi Annan, in Ankara after a weekend mission to Damascus, acknowledged that a settlement in Syria would "not be easy" but renewed his demand for an immediate halt to the "unacceptable" killings of civilians.
UN chief Ban Ki-moon called on Damascus to respond "within the next few days" to the set of concrete proposals which his predecessor handed to Assad in their talks on Saturday and Sunday.
The grisly murders in Syria's third-largest city Homs came less than two weeks after regime troops stormed its rebellious Baba Amr neighbourhood, following a month-long bombardment in which activists say 700 people were killed.
Activist Hadi Abdallah 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.
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.
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.
The television said the weekend killings were a clear ploy by "armed terrorist gangs" to grab the spotlight ahead of the meeting of major powers in New York.
"We are used to them committing more crimes before meetings of the UN Security Council," it said.
But at a meeting in Istanbul, the opposition Syrian National Council ridiculed the claims of the government.
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"Each time something happens in Syria, the regime fabricates its own story," senior SNC official George Sabra said.
"The videos and the pictures all show clearly, as clear evidence, that it is the regime which is committing these massacres against the civilians."
Sabra called for "urgent Arab and international military intervention," adding that should include both the creation of a "no-fly zone" over all of Syria and "strikes" against the Syrian armed forces.
At the Security Council meeting in New York, the Russian foreign minister maintained his argument against "unilateral" UN action.
Change in the Arab world "must not be achieved by misleading the international community or manipulating the Security Council," Lavrov said.
But US Secretary of State Hillary Clinton said after the meeting that she believed the Russian minister had heard "how strong the feelings are."
"We expect all nations including Russia and China to join us now in pressing the Assad regime to silence the guns," she said.
"How cynical that, even as Assad was receiving former secretary general Kofi Annan, the Syrian army was conducting a fresh assault on Idlib and continuing its aggression in Hama, Homs, and Rastan?" she told the meeting.
Fresh military action was reported in all four protest centres on Monday, with the Britain-based Observatory reporting 27 people killed nationwide, 12 of them civilians.
In Ankara, Annan acknowledged that the situation in Syria was "complex" but he was confident that talks for a settlement of the year-old crisis, which human rights monitors say has claimed more 8,500 lives, would eventually succeed.
"We will launch a political process and we will reach a settlement," Anatolia news agency quoted him as saying.
The head of a UN human rights probe called for immediate access by relief agencies to civilians trapped by the fighting in besieged protest cities saying they were facing a "desperate situation."
"The intensification of armed confrontations has widened the trail of suffering," the president of the UN Human Rights Council inquiry, Paulo Pinheiro said.
"Unimpeded humanitarian access should be granted as a rule, rather than an exception," he added.