At least 69 people were killed in an explosion in Syria's central city of Hama on Thursday, al Jazeera television reported, citing activists.
The activists said the blast was caused by government shelling.
State media said the blast hit a house used as a bomb factory by "armed terrorist groups."
The Britain-based Syrian Observatory for Human Rights has asked UN monitors in the area to investigate.
Hama has been a focus of the 13-month uprising against President Bashar al-Assad.
The violence comes despite a UN-brokered ceasefire which has been in place since April 12 as part of a peace plan put forward by UN envoy Kofi Annan.
Both sides have accused the other of violating the truce.
Thirteen children and 16 women were among those killed in the blast, which occurred in Hama's southern Masha at-Tayyar district, activists said.
The Syrian National Council, the country's main opposition group, has called for an emergency UN Security Council meeting "so that it can issue a resolution to protect civilians," the BBC reported.
Activists say Syrian forces shelled another Hama district on Tuesday, killing at least 20 people and injuring more than 60 others.
Syria says it is complying with Annan's peace plan, but the UN says the violence is continuing and agreed the deployment of a 300-strong observer team there.
"Today, the implementation of the Annan plan is difficult, but it is being implemented," Qadri Jamil, a representative of Syria's Popular Front for Change and Liberation, said during talks in Moscow on Thursday.
Meanwhile, French Foreign Minister Alain Juppe said the UN Security Council may have to consider military action in Syria if Annan's peace plan fails.
Annan's report on May 5 would be "a moment of truth," he said. "Either this mediation is working, or it isn't."