UN peacekeeping chief Herve Ladsous said on Tuesday that Syrian troops have kept heavy weapons in cities, and that both the government and rebels have violated the putative truce that went into effect 12 April.
He also said UN members had so far only offered only 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 that 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.
Ladsous said 24 monitors were currently in place.
"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, added Ladsous, a UN under secretary general.
"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.
More than 30 people, the majority of them civilians, were killed in violence across the country on Tuesday, monitors said.
Meanwhile Human Right Watch accused the regime on Wednesday of committing atrocities in Idlib province shortly before the truce went into effect.
"Syrian tanks and helicopters attacked one town in Idlib after another," Anna Neistat, associate director for programmes and emergencies at HRW, said in the report entitled "'They Burned My Heart': War Crimes in northern Idlib during Peace Plan negotiations."
"It was as if the Syrian government forces used every minute before the ceasefire to cause harm," she added.
The New York-based organisation accused regime forces of summary executions, arbitrary detentions and burning and destruction of civilian property.
In some of the incidents recorded by the global rights watchdog, children were executed by regime forces.
"The security forces also arbitrarily detained dozens of people, holding them without any legal basis," HRW said.
According to one eyewitness account published by HRW, the mother of Mohammed Saleh Shamrukh, an anti-regime protester from Saraqeb, in Idlib province, had to watch regime forces take him away.
"I didn't say goodbye so as to not make him sad. He didn't say anything either. When they left, the soldiers said I should forget him," she said.
Shamrukh was executed on 25 March.
Another woman recounted how regime forces entered her home in the town of Taftanaz searching for her husband.
"They put a Kalashnikov to my head and threatened to kill us all if my husband did not come home," she said.
"Then an officer told a soldier to get petrol and told the children that he would burn them like he would burn their father because he is a terrorist."
She said she was finally allowed to leave the house before it was burned down on 4 April along with the houses of her five brothers-in-law.
HRW said that during the April 3-4 attack on Taftanaz, northeast of Idlib city, 19 members of the Ghazal family, including two under the age of 18, were executed by regime forces. Nine males were shot in the head or back.
A 76-year-old man identified as Ali Maassos was also gunned down by regime forces along with his wife Badrah as they tried to flee Taftanaz in a pick-up truck with other residents, the report said.
"The circumstances of these cases indicate that government forces failed to distinguish between civilians and combatants and to take necessary precautionary measures to protect civilians," HRW said.
The rights group has previously condemned abuses by anti-regime fighters in Syria. But "these abuses by no means justify ... violations committed by the government forces."
The truce is part of a peace plan brokered by United Nations-Arab League envoy Kofi Annan.
The plan calls for 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.
According to the UN, more than 9,000 people have been killed in Syria since an anti-regime uprising broke out in March last year.