Peace envoy Kofi Annan said Syria had "flouted" Security Council resolutions with mass killings in Treimsa village, as UN chief Ban Ki-moon demanded that the Council act to stop the bloodshed.
Activists said about 150 people were killed in what the Syrian opposition charged was a massacre by regime forces on Thursday but which the government claimed was a clash with rebels.
Ban demanded that the UN Security Council act on the 16-month-old conflict, warning that any failure would be giving "a license for further massacres".
Ban and Annan, the UN-Arab League envoy, expressed outrage at the killings and to make an "urgent call" for increased pressure on President Bashar al-Assad, whose regime launched a brutal crackdown on dissent in March 2011.
"I call upon all member States to take collective and decisive action to immediately and fully stop the tragedy unfolding in Syria. Inaction becomes a license for further massacres," Ban said in a hard-hitting statement aimed at the council.
The Syrian Observatory for Human Rights said shelling and fighting in the central province of Homs killed at least five people on Saturday, including a pregnant woman and two rebels.
The woman was killed in shelling of Qusayr city, while the rebel fighters died during clashes in the Homs city neighbourhoods of Sultaniyeh, Qarabis and Khaldiyeh, it said.
Elsewhere, helicopters backed by armoured vehicles attacked Khirbet Ghazaleh in the southern province of Darra, a main hotbed of dissent against President Bashar al-Assad's regime, it added without providing any casualty figures.
On Friday, violence across Syria killed at least 118 people, including dozens of civilians when government forces fired on anti-regime protests, said the Observatory.
The massacre in Treimsa has added new urgency to deadlocked Security Council negotiations on a Syria resolution. Russia has rejected western demands for non-military sanctions to be threatened to back Annan's peace efforts.
Washington has called for a ceasefire to let UN Supervision Mission in Syria (UNSMIS) observers into Treimsa in the central province of Hama.
"The use of artillery, tanks and helicopters, which has been confirmed by UNSMIS, is a violation of the Syrian government's obligations and commitment to cease the use of heavy weapons in population centres," Annan said in a letter to the 15-nation council obtained by AFP.
"Tragically, we now have another grim reminder that the council's resolutions continue to be flouted."
He again called on the council, which is debating rival draft resolutions on Syria, to "send a message to all that there will be consequences for non-compliance".
Western nations have proposed a resolution that would impose sanctions on Assad for not ending the use of heavy weapons in the conflict, which rights activists say has left more than 17,000 people dead.
But Russia has rejected any use of sanctions.
"I condemn this outrageous escalation of violence in the strongest possible terms, and fully concur with the views expressed by the joint special envoy," Ban said in a note to the council with Annan's letter.
French President Francois Hollande called on Russia and China not to oppose UN sanctions against Syria following the latest massacre, saying it would "only result in chaos and war in Syria at the expense of (your) own interests."
In Brussels, EU foreign policy chief Catherine Ashton, putting the death toll in Treimsa at 200, denounced the killings as "an atrocity" and "a blatant violation" of Annan's six-point peace plan, which has not been observed.
