Syrian forces raided the city of Hama after shelling the rebel city of Homs and attacking other towns before U.N. truce monitors started their mission Monday.
An unidentified military source told the official Syrian Arab News Agency that "armed terrorist groups have hysterically escalated" their campaign against Syrian forces and civilians.
"Out of their duty to protect the security of the homeland and citizens, the authorities will prevent these armed terrorist groups from continuing their criminal aggressions and acts of killings and sabotage against the citizens and their properties," the source was quoted as saying.
Former U.N. Secretary-General Kofi Annan brokered a cease-fire last month that set a Thursday deadline for an end to major military operations.
An advance team of six unarmed monitors arrived Monday in Syria to observe the terms of the cease-fire. Roughly two dozen more are expected by the end of the week. A team of 250 would be sent to Syria following further negotiations with Damascus.
The U.N. Security Council, in a resolution authorizing the advance team, said the purpose of the mission is to "to liaise with the parties and to begin to report on the implementation of a full cessation of armed violence in all its forms by all parties."