More than 20 people were killed on Monday in blasts targeting security buildings in the city of Idlib, northwest Syria, the Syrian Observatory for Human Rights said.
The majority of those killed were members of the security forces, the Britain-based group said.
A powerful blast, probably a car bomb, was also reported near the capital Damascus, causing casualties, the Observatory added.
"A strong explosion shook the suburbs of Qudsiya and it appears it was a car bomb," it said. "Initial reports indicate there are casualties."
The blasts took place in spite of a UN-backed ceasefire that came into effect April 12 but has failed to stop the violence.
Veteran peacekeeper Major General Robert Mood urged all sides on Sunday to abide by the ceasefire as he arrived in Damascus to take command of the UN military observer mission overseeing the truce.
The peace plan brokered by UN-Arab League envoy Kofi Annan calls for a commitment to stop all armed violence, a daily two-hour humanitarian ceasefire, media access to all areas affected by the fighting, an inclusive Syrian-led political process, a right to demonstrate and the release of arbitrarily detained people.