Twin blasts targeting security buildings killed more than 20 people in the northwest Syrian city of Idlib, a monitoring group said, as the chief UN monitor presses both sides to end more than 13 months of violence.
The violence, a day after the arrival of chief monitor Major General Robert Mood, was sure to put further strain on a putative 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 by "suicide bombers."
Syrian television put the death toll at nine, among them civilians, and said around 100 people were wounded in the two blasts in residential areas.
It broadcast footage of blood stains on the ground, and groups of angry people denouncing the violence and expressing support for President Bashar al-Assad's regime.
"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 explosion.
Hours later a third blast rocked the city's university neighbourhood, and the Britain-based Observatory said: "There are reports of wounded."
SANA said UN observers had gone to visit the scene of the morning explosions.
A powerful blast, probably a car bomb, was also reported in the suburb of Qudsiya near Damascus, causing an unknown number of casualties, the Observatory added.
The explosion targeted a military vehicle, said Abdel Rahman, causing an undetermined number of casualties, including civilians living nearby being wounded.
At the same time, the Observatory said six civilians were killed in various parts of the country.
Troops were also reported searching for deserters in Kfar Nabal in Idlib province, and making arrests in the Deir Ezzor town of Quriya.
On Friday, a suicide car bomb in central Damascus killed 11 people.
Anti-regime activists have accused the government of being behind the series of explosions, while the authorities blame "terrorists."
The Syrian National Council, the main opposition group, said the RPG attacks in Damascus 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," a statement said, calling for "an international commission of inquiry to uncover who was behind the explosions."
General Mood, a veteran peacekeeper, urged all sides on Sunday to abide by the ceasefire as he arrived 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 stressed that 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," the Norwegian said.
Deputy Foreign Minister Faisal Meqdad told Mood that Syria "will confront the actions of armed bands and those who support them, especially after the unprecedented escalation by the terrorist bands since the first observers arrived," SANA said.
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 began last year.
Washington, meanwhile, renewed calls for the Syrian authorities to release human rights activist and journalist Mazen Darwish.
"The Syrian government has held Darwish incommunicado since security forces raided his offices on February 16," State Department spokeswoman Victoria Nuland told reporters.
"And we are very concerned that he could be the subject of torture, abuse or other inhumane treatment.
"We take this opportunity to call on the Syrian government to release Darwish and other journalists that it has imprisoned, as well as all political prisoners, in keeping with the six-point Annan plan."