Twin blasts targeting security buildings killed more than 20 people in the northwest Syrian city of Idlib on Monday, as an explosion was also reported in the capital, a monitoring group said.
The violence a day after the arrival of the chief of a United Nations monitoring mission was sure to put further strain on a UN-backed ceasefire that went into effect on April 12 but has failed to take hold fully.
Most of those killed in Idlib were members of the security forces, the Syrian Observatory for Human Rights said.
"The blasts targeted two security headquarters, one housing air force intelligence, and the other military intelligence," Observatory chief Rami Abdel Rahman told AFP.
State news agency SANA said "terrorists" were behind the attacks that were carried out by "suicide bombers."
Syrian television put the death toll at nine, among them civilians, and said around 100 people were also wounded in the two blasts in residential areas of the city.
It broadcast footage of bloodstains on the ground in one neighbourhood, and groups of angry people denouncing the violence and expressing support for the regime of President Bashar al-Assad.
"Is this the freedom they want?" shouted one man, standing near a woman who was carrying a child with blood running down his forehead.
One building appeared in ruins and cars nearby were flattened by the force of the explosion.
Hours later a third blast rocked the university neighbourhood of Idlib, and the Britain-based Observatory said: "There are reports of wounded."
A powerful blast, probably a car bomb, was also reported in the suburb of Qudsiya near the capital Damascus, causing an unknown number of casualties, the Observatory added.
The explosion targeted a military vehicle, said Abdel Rahman.
"Initial reports indicate there are casualties," he added. "But we cannot yet confirm the number of victims."
An unknown number of civilians living in houses near the site of the explosion were wounded, he said.
Overnight, a rocket-propelled grenade hit the Central Bank in the capital, state media said, adding that an "armed terrorist group" also carried out a second RPG attack on a police patrol in front of a hospital in the Damascus area of Rokn Eddin. Four police were wounded.
On Friday, a suicide car bomb in the heart of the capital killed 11 people.
Anti-regime activists have accused the government of being behind the series of explosions, while the authorities say "terrorists" are responsible.
The Syrian National Council, the main opposition group, said in a statement that the RPG attacks in the capital were "another trick" by the regime to justify its continued crackdown against a revolt that began in March last year.
"The Assad regime is trying in various ways to mislead and distract (UN) observers in order to prevent them carrying out their work," the statement said, also calling for "an international commission of inquiry to uncover who was behind the explosions."
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 detainees.
"To achieve the success of the Kofi Annan plan, I call on all sides to stop violence and help us continue the cessation of armed violence," Mood told reporters.
"We will work for the full implementation of the six-point Annan plan which the Syrian government agreed to.
"To achieve this, we now have 30 monitors on the ground, and in the coming days we will double this figure," he said, adding that the number would "rapidly" increase to 300.
Mood, a 54-year-old Norwegian who negotiated the conditions for the deployment of the advance team, was head of the UN Truce Supervision Organisation, which monitors Middle East truces, from 2009 until 2011.
He stressed the monitors need the cooperation of all parties to achieve their mission: "The observers can't solve all problems in and of themselves... All sides must stop violence and give the process a chance."
At least 70 people, among them 47 civilians, were killed nationwide at the weekend, monitors said.
A spokesman for the advance team of observers said they had set up base in major troublespots, including Idlib, central Homs and Daraa in the south.
The United Nations estimates that more than 9,000 people have been killed since the revolt against Assad's regime broke out in March last year.
The uprising started as a popular revolt but has since transformed into an insurgency.