The UN peacekeeping chief accused both the Syrian regime and its foes of violating a ceasefire accord, as nine members of the same family and 12 troops were among 34 people killed on Tuesday.
Nine members of a single family were among the 10 killed in regime bombardment of a village in 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 that four women and two children were among the dead.
In Deir Ezzor province in the northeast, 12 soldiers were killed in clashes with rebel fighters, said the watchdog. In other violence across the country, at least another 12 people died, including two rebel fighters.
On the international front, US President Barack Obama gave the US Treasury authority to tighten sanctions against Syria and Iran by going after foreign firms or individuals that violate existing measures.
The move, contained in an executive order, will allow the Treasury to publicly identify those engaging in "evasive and deceptive activities" and bar them from access to the US financial and economic sectors, officials said.
For his part, UN peacekeeping chief Herve Ladsous said Syrian troops have kept heavy weapons in cities, and that both the government and rebels have violated the truce.
He also said UN members had so far only offered 150 military observers for the 300-strong planned force and that Syria had refused visas for three proposed monitors.
But Syrian foreign ministry spokesman Jihad Makdisi denied visa requests had been turned down and said the two sides had agreed on the nationalities who could operate in Syria.
"We agreed with the UN negotiating team that nationalities of observers to be mutually agreed upon... So there is no refusal per se... There are far more than 110 nationalities that can easily work in Syria," he told AFP.
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Ladsous said 24 monitors were currently in place, despite earlier reports that they number 30.
"Regarding the heavy weapons, yes, our military observers do see a number of APCs (armoured personnel carriers), for instance; they see a number of Howitzers and other military equipment in most places where they are," he said.
Syria has told the monitors that the armoured carriers have been disarmed but this has not been verified, Ladsous added.
Ladsous, a UN under secretary general, said government forces and opposition groups have broken the truce.
"The important fact is that violations do come from both sides," he said while refusing to say whether one side had committed more breaches.
"All the parties need to take further steps to ensure a cessation of violence in all its forms," he said.
At the moment, two observers are 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 others remain based in the capital.
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 and wounded three people.
The Al-Nusra front said 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."