The UN Security Council has condemned "in the strongest possible terms" the Syrian government for the Houla massacre in which at least 108 people were killed.
A statement agreed by the 15-nation council, including Syrian ally Russia, said the attacks "involved a series of government artillery and tank shellings on a residential neighbourhood".
It also once again demanded that President Bashar al-Assad withdraw heavy weapons from Syrian towns.
Images of bodies lying side-by-side after the onslaught triggered shock around the world and underlined the failure of a six-week-old UN ceasefire plan to stop the violence.
Western and Arab states opposed to Mr Assad put the blame for the deaths squarely on the government.
Russia, which along with China had earlier vetoed Security Council resolutions calling for tougher action, said the "tragic" events in Syria deserve condemnation and called for a UN assessment of the violence there.
In his only public comments so far, UN observer mission General Robert Mood called the killings "a very tragical expression" of the situation in Syria, but refrained from apportioning blame.
"For myself, I have had patrols on the ground all the day yesterday afternoon and today we are gathering facts on the ground and then we will draw our own conclusions," General Mood told the BBC in a telephone interview.
"The sight like the one that now has been played out in Houla, it hits the stomach, it's really an attack on the future of the Syrian people. There are still those in Syria who believe the use of violence and quite deplorable use of violence serves their own self-interest," General Mood said.
Syrian authorities blamed "terrorists" for the massacre. It is among the worst carnage in the 14-month-old uprising against Mr Assad, which has cost about 10,000 lives.
"Women, children and old men were shot dead. This is not the hallmark of the heroic Syrian army," Foreign Ministry spokesman Jihad Makdesi told reporters in Damascus.
The White House said it was horrified by credible reports of brutal attacks on women and children in Houla.
"These acts serve as a vile testament to an illegitimate regime that responds to peaceful political protest with unspeakable and inhuman brutality," a White House spokesman said.
The Gulf Cooperation Council of Sunni-led monarchies accused Assad's soldiers of using excessive force and urged the international community to "assume its responsibilities to halt the daily bloodshed in Syria."
Tánaiste and Minister for Foreign Affairs Eamon Gilmore described the incident as ''a deplorable act and a clear crime against humanity.''
EU foreign policy chief Catherine Ashton spoke of a "heinous act perpetrated by the Syrian regime against its own civilian population" in a statement. The head of the European Parliament said it could amount to a war crime.
Meanwhile today, at least 30 people were killed when Syrian army tanks shelled residential neighbourhoods in the city of Hama that have been serving as bases for rebel attacks against loyalist forces, opposition activists said.
The reports could not be verified independently as the media are not allowed to operate in Syria.
Although the ceasefire plan negotiated by Mr Annan has failed to stop the violence, the United Nations is nearing full deployment of a 300-strong unarmed observer force meant to monitor a truce.
The plan calls for a truce, withdrawal of troops from cities and dialogue between government and opposition.
Syria calls the revolt a "terrorist" conspiracy run from abroad, a veiled reference to Sunni Muslim Gulf powers that want to see weapons provided to the insurgents.
The UN has accused Mr Assad's forces and insurgents alike of grave human rights abuses, including summary executions and torture.
