TV reports said two suicide bombings had killed eight people, while the Syrian Observatory for Human Rights said more than 20 had died in attacks targeting the security forces.
The UN is currently deploying monitors to the country to oversee a fragile peace plan.
Thirty will be in place soon but the UN warns it will need many more.
The Observatory said the Idlib bombs had exploded next to the Air Force Intelligence headquarters and the Military Intelligence building, with most of the casualties security personnel.
State TV said there had been “two terrorist suicide bombs”, killing eight and wounding dozens – both civilians and security personnel.
It showed scenes of devastation, with big buildings partially collapsed, and streets full of rubble and wrecked vehicles. It also showed pools of blood left behind by some of the casualties. The blasts threw debris hundreds of metres.
The Observatory also said there had been a powerful blast near the capital Damascus, causing casualties, but this has not been independently confirmed.
State TV also said there had been a rocket-propelled grenade attack by three men on the Syrian Central Bank in Damascus overnight, but again this has not been verified.
Activists said that one of Monday’s blasts was only about 200m from the monitors’ hotel.
The head of the UN observer mission to Syria, Maj Gen Robert Mood, has arrived in Syria and will be followed by another 30 observers in the coming days, doubling the size of the mission.