UN chief Ban Ki-moon told the Security Council that according to preliminary evidence, troops had surrounded al-Qubeir and militia entered the village and killed civilians with "barbarity".
Damascus denied responsibility and blamed foreign-backed "terrorists," as it has done repeatedly in the past.
But the findings have prompted Western governments to launch a new push for harsher sanctions against the Syrian regime.
Diplomats in New York said Britain, France and the United States would quickly draw up a Security Council resolution proposing the sanctions. "We will move fast to press for a resolution," said one UN diplomat.
Activists said the shelling attack in Deraa, where the first seeds of the Syrian uprising were sown in March 2011, could be revenge for new offensives launched by the rebel Free Syrian Army in the area.
"Rebels have been attacking checkpoints in many areas across the country in the evenings," said the Syrian Observatory chief Rami Abdel Rahman.
Across the country rebel fighters have grown in confidence in the last weeks. Spurred by anger at the massacres in al-Houla and al-Qubeir rebel fighters in Idlib, Deraa, Hama, Latakia and the capital have attacked Syrian military checkpoints, killing hundreds of Syrian government troops.
"They think that this is the beginning of the end. The FSA are so confident now, they think it is a matter of time before the army crumbles," said Dr Mousab Azzawi from the Syrian Network for Human Rights.