Peace envoy Kofi Annan said Friday Syria had "flouted" Security Council resolutions with mass killings in Treimsa village which UN chief Ban Ki-moon called an "outrageous escalation".
Activists said about 150 people were killed in what the Syrian opposition charged was a massacre by regime forces but which the government claimed was a clash with rebels.
Washington called for a ceasefire to let UN Supervision Mission in Syria (UNSMIS) observers into Treimsa in the central province of Hama.
News of Thursday's killings drew sharp international condemnation, with calls for tougher UN action in the 16-month uprising against President Bashar al-Assad.
"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 a resolution 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. But Russia has rejected the 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.
In Brussels, EU foreign policy chief Catherine Ashton, putting the death toll at 200, denounced the Treimsa killings as "an atrocity" and "a blatant violation" of Annan's six-point peace plan.
"I am deeply shocked by reports of the ruthless killing of at least 200 men, women and children," she said in a statement.
"The regime's use of heavy weaponry, including artillery and helicopters, which has been confirmed by UN observers, is a blatant violation of its obligations under the Annan plan."
The Syrian Observatory for Human Rights said regime forces used tanks and helicopter gunships backed by militias in the Treimsa killings.
Rebel leader Abu Mohamad, whose fighters are based near the village, said more than 200 people were slaughtered.
The Observatory was more cautious, saying "several dozen rebel fighters were among those killed," adding that only around 40 of the dead had been identified, while 30 were burned and 18 were "summarily executed".
"Some are estimating higher numbers, but even at around 150, especially considering how small the town is, this might be the biggest massacre committed in Syria since the start of the revolution," Observatory head Rami Abdel Rahman told AFP.
An activist at the anti-regime Sham News Network said most of the dead were rebels and that the bloodbath took place when regime forces retaliated after the rebel Free Syrian Army (FSA) attacked a convoy.
"At this stage, though we do not yet have the final count, the number of civilians killed by shelling is not more than seven," the activist who identified himself as Jaafar told AFP.
"The rest were members of the Free Syrian Army."
Syria's military said the army killed "many terrorists" in Treimsa, but no civilians.
"Army units carried out a special operation," a spokesman said, "targeting armed terrorist groups and their leadership hide-outs."
It "eradicated the terrorist groups' lairs and killed many terrorists," he said, adding that "the terrorists were dealt with, while there were no civilian victims".
The spokesman said that after the operation, "our armed forces... carried out a search operation, and found the bodies of a number of people whom the armed terrorist groups had abducted and assassinated".
Treimsa, which had a population of 7,000, "is empty now. Everyone is dead or has run away," an activist calling himself Abu Ghazi said.
Treimsa is near Al-Kubeir, where at least 55 people were killed on July 6, according to the Observatory. Like Al-Kubeir, Treimsa is a majority Sunni village situated near Alawite hamlets.
Assad belongs to the Alawite community -- an offshoot of Shiite Islam -- although most Syrians are Sunni.
If confirmed, the 150-person toll would exceed that of a massacre at Houla on May 25, when a pro-Assad militia and government forces were accused of killing at least 108 people.
US Ambassador to the UN Susan Rice said in a Twitter message that the killings "dramatically illustrate the need for binding measures on Syria" by the Security Council.
The council's talks have so far failed to narrow differences.
Britain, France, the United States, Germany and Portugal have proposed a resolution that would impose sanctions on Syria if Assad's regime does not halt violence and works with the international peace plan.
Russia is proposing a rival resolution that renews the mandate of the UN mission in Syria, which ends on July 20, and has said sanctions are unacceptable.
US Secretary of State Hillary Clinton called for a ceasefire.
"We call for an immediate ceasefire in and around Hama to allow the UN observer mission to enter Treimsa. Those who committed these atrocities will be identified and held accountable," she said in a statement.
Major General Robert Mood, head of the UN mission, has said his team was ready to go to Treimsa if a ceasefire is in place.
The opposition Syrian National Council called for a binding Security Council resolution.
"To stop this bloody madness which threatens the entity of Syria, as well as peace and the security in the region and in the world, requires an urgent and sharp resolution of the Security Council under Chapter VII (of the UN Charter) which protects the Syrian people," it said.
Chapter VII allows for punitive measures against regimes considered a threat to the peace, including economic sanctions and military intervention.
The Syrian Observatory reported at least 75 people killed across Syria on Friday -- 25 civilians, 24 soldiers and 26 rebels. The watchdog has said more than 17,000 people have been killed since the uprising began in March 2011.