Ban Ki-moon, the UN secretary-general, has said that massacres of civilians, such as the one perpetrated in Houla last weekend, could plunge Syria into a devastating civil war.
Ban cited fears raised on Tuesday by Kofi Annan, the UN-Arab League envoy, that Syria may have already reached a "tipping point" following the slaughter of 108 people in Houla last Friday.
"The massacre of civilians of the sort seen last weekend could plunge Syria into catastrophic civil war - a civil war from which the country would never recover," he said in Istanbul.
Meanwhile, state media reported that 500 prisoners who were arrested on suspicion of involvement in the 14-month-old revolt had been released, two days after Annan urged President Bashar al-Assad to free detainees.
"Five hundred people involved in the events taking place in Syria, without blood on their hands, have been released", a newsflash on state television read.
International pressure and a six-point peace plan drafted by Annan, which calls on both the government and opposition forces to cease fire, have failed to end the conflict which has claimed thousands of lives.
Hillary Clinton, US secretary of state, said on Thursday that every day of slaughter in Syria is strengthening the case for tougher international action, yet stressed that military intervention would need international support, including from Syrian ally Russia.
"The continued slaughter of innocent people, both by the military and by militias supported by the government and increasingly by the opposition... could morph into a civil war in a country that would be riven by sectarian divides, which then could morph into a proxy war in the region," she said.
In-depth coverage of escalating violence across Syria
Speaking to Danish university students, Clinton noted that UN and international backing made possible last year's international coalition against Muammar Gaddafi in Libya but said Russia and China are standing in the way of similar action in Syria.
She also suggested that a military effort against Assad's government would be far more difficult.
She said the US would continue pressing its case to Russia, which has promised to block any moves at the UN to obtain a military mandate for intervention.
Another key ally of Syria, China, said on Thursday that a resolution of the Syria conflict needed more time and urged support and patience for Annan's peace plan.
"The situation in Syria currently is very complex and grave, at the same time Annan's mediation efforts have been productive," Liu Weimin, China's foreign ministry spokesman, said.
"Due to the extreme complexity of the Syrian issue, it will take time to reach a resolution ... we should not lose our confidence and patience and we should not easily admit failure. We must be supportive of Annan's mediation efforts."