UN chief Ban Ki-moon said it was vital that government and opposition alike cooperate fully with a hard-won UN observer force as it fans out in its mission to shore up the ceasefire agreement that took effect on April 12.
But despite his appeal, human rights monitors reported no let-up in the violence, a day after a series of blasts condemned by Ban as "terrorist bomb attacks" killed more than 20 people.
Nine members of a single family were among the 10 killed in the regime bombardment of a village in northwestern Idlib province, said the Syrian Observatory for Human Rights.
A mortar round slammed into their home in Mashmashan village near the town of Jisr al-Shughur, the watchdog said, adding four women and two children were among the dead.
Elsewhere in the same province, random gunfire killed a 13-year-old boy in the town of Maaret al-Numan.
Outside Damascus, gunfire from a minibus belonging to the security forces killed an 80-year-old man in the town of Qatna, while regime shelling killed a civilian in a village of central Hama province, said the Observatory.
In Deir Ezzor province in the northeast, 12 soldiers were killed in clashes with rebel fighters, said the watchdog.
One civilian was also killed during the fighting in the Busayra area of the province, in which troops responded with missile and heavy machinegun fire, it added.
Ban called on all sides to work with the UN observer mission as it expands from the current advance party of 30 to a promised full complement of 300 in the coming days and weeks.
"While noting improvements in areas where UN monitors are deployed, the secretary general remains gravely concerned by reports of continued violence, killing and abuses in Syria in recent days," a statement said.
He called "for armed violence in all its forms by all parties to cease immediately and full cooperation of all parties with the work of UN Supervision Mission in Syria as it expands its presence on the ground."
Two observers are already deployed in Idlib, a province bordering Turkey where rebel fighters of the Free Syrian Army have been active. There are none in Deir Ezzor.
Two observers each have been deployed in three other protest centres -- the flashpoint central cities of Hama and Homs, and Daraa province, south of Damascus and cradle of the uprising against President Bashar al-Assad's regime that broke out in March last year.
The other 22 remain based in the capital.
In Monday's violence, bombings rocked Idlib city and the Damascus suburbs, with the deadliest hitting security buildings in Idlib.
"The blasts targeted two security headquarters, one housing air force intelligence and the other military intelligence," Observatory chief Rami Abdel Rahman told AFP.
The official SANA news agency said "terrorists" were behind the attacks by "suicide bombers."
Meanwhile, an Islamist group that has claimed several bomb attacks in Syria said it was responsible for a blast in Damascus last week that targeted security forces and an Iranian cultural centre.
The Al-Nusra front said in a statement posted online that one of its members had attached an explosive device to an army vehicle and that it detonated at the cultural centre in Marjeh Square, "hitting two targets in the process."
Police said three people were injured in the April 24 blast.
In his statement, the UN chief condemned what he called "terrorist bomb attacks" in Idlib and Damascus.
But the opposition Syrian National Council again accused the authorities of orchestrating the violence.
"The Assad regime is trying in various ways to mislead and distract (UN) observers in order to prevent them carrying out their work," the SNC said, calling for "an international commission of inquiry to uncover who was behind the explosions."