A Syrian artillery barrage has killed more than 90 people, including dozens of children.
It is the worst violence since the start of a UN peace plan to staunch the flow of blood from Syria's uprising, activists said today.
The bloodied bodies of children were shown in footage posted to YouTube purporting to show the victims of the shelling in the central town of Houla yesterday.
The reports could not be confirmed independently as Syria has banned foreign media.
French Foreign Minister Laurent Fabius condemned the violence as a "massacre", and said he wanted to arrange a meeting in Paris of the Friends of Syria, a group that brings together Western and Arab countries keen to remove President Bashar al-Assad.
Syrian state television aired some of the footage disseminated by activists, blaming "terrorist" gangs.
It also showed video of bodies with what looked like gunshot wounds to the head, sprawled on bloodstained mattresses.
A British-based opposition group, the Syrian Observatory for Human Rights, said residents of Houla were fleeing in fear of more shelling.
It said one person was killed in the northern town of Saraqeb when security forces opened fire on a protest against the killing.
Activists distributed footage appearing to show similar protests in Aleppo, the largest city in the north.
A member of the fragmented exile group that says it speaks for Syria's political opposition said Assad's forces had killed "entire families" in Houla in addition to the shelling.
"The Syrian National Council [SNC] urges the UN Security Council to call for an emergency meeting ... and to determine the responsibility of the United Nations in the face of such mass killings," SNC spokeswoman Bassma Kodmani said.
UN Secretary-General Ban Ki-moon said yesterday that recent bomb attacks may have been the work of "established terrorist groups" and urged states not to supply arms to either the government or rebel forces.
"Those who may contemplate supporting any side with weapons, military training or other military assistance, must reconsider such options to enable a sustained cessation of violence," he told the Security Council in a letter.
The United Nations has accused Assad's forces and insurgents alike of grave human rights abuses, including summary executions and torture.
Mr Ban has also expressed fear that Syria's conflict will destabilise neighbouring Lebanon, whose delicate sect-based politics has been shaken by tensions among Lebanese foes and friends of the uprising in Syria.