Government media say a massive blast in Syria's central city of Hama has killed at least 16 people and was caused by rebels who mishandled explosives.
Antiregime activists, however, said that they are not sure what caused the blast on April 25, and put the death toll at more than 50.
The Syrian Observatory for Human Rights, an activist group, has asked UN monitors to investigate.
Witnesses initially said the blast in Hama was caused by government shelling.
Opposition activists, meanwhile, said at least seven people were killed by the military across the country on April 26.
The violence in Syria has prompted calls for a faster deployment of UN observers to monitor the fragile cease-fire in effect since April 12, which was brokered by international envoy Kofi Annan.
Only 10 out of 300 monitors are currently in the country.
Based on reporting by AFP and dpa