UN Secretary-General Ban Ki-moon has urged the Security Council to take "collective action" to end the violence in Syria, one day after an estimated 200 civilians were killed in the central town of Tremseh.
UN monitors in Syria blamed the deaths on Syrian government forces using heavy weaponry.
UN-Arab League envoy Kofi Annan also urged the Security Council to insist its resolutions be implemented.
The United States and the European Union also condemned the attack.
U.S. Secretary of State Hillary Clinton said there is "indisputable evidence" the Syrian regime has "deliberately murdered civilians" in Tremseh. She called for an immediate cease-fire and for UN observers to be allowed to enter the site.
Meanwhile, the Russian Foreign Ministry has condemned the attack. The ministry blamed the violence on "forces that do not seek peace" and which are trying to "cultivate seeds of interethnic violence."
Moscow -- which is the closest ally of Syrian President Bashar al-Assad -- has often accused the Syrian opposition of fomenting violence in order to draw Western powers into the conflict.
Based on reporting by AFP, AP, and Interfax