A number of people have been killed in blasts in the north-western Syrian city of Idlib, activists and state TV say.
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.
UN mission's plea
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 "two terrorist suicide bombs" in Hananu Square on Carlton Street had killed eight and wounded 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.
One activist told Associated Press news agency the blast sites were several hundred metres apart and the bombs went off within five minutes of each other shortly after daybreak.
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.
The BBC's Jim Muir in Beirut says Idlib has been known for its defiance to the government, but it has been relatively calm in recent days because there have been two UN monitors stationed there.
Activists said that one of Monday's blasts was only about 200m from the monitors' hotel, with some reports saying it sustained damage.
The state-run Sana news agency said the monitors later toured the site of the bombings.
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.
The UN has approved up to 300 observers under a peace plan brokered by UN and Arab League envoy Kofi Annan.
However, Gen Mood warned that "10 unarmed observers, 30 unarmed observers, 300 unarmed observers, even 1,000 unarmed observers cannot solve all the problems".
He said: "To achieve the success of Kofi Annan's six-point plan, I call on all to stop the violence and to help us on a continued cessation of armed violence in all its forms."
At least 500 people have died since the ceasefire was agreed on 12 April, activists say.
The government and opposition have blamed each other for the violence.
The Local Coordination Committees said the attacks were government "tricks" that "no longer fool anyone".
It said: "The regime has resorted to these escalations every time there is political movement at the Arab, regional or international level to find a political solution."
On Thursday UN Secretary General Ban Ki-moon warned that Syrian's government was "in contravention" of the UN and Arab League peace plan, and demanded that Damascus comply with its terms without delay.