Syrian troops have killed 28 civilians in the city of Hama, according to monitors, as UN military observers tour protest centres near Damascus.
The persistent violence - 11 days into a ceasefire - has sparked criticism from opposition activists of the fledgling UN mission.
Speaking on UN Television, spokesman Ahmad Fawzi also cited "credible reports" that said Syrian forces were intimidating and, in some cases, killing people who speak to UN truce mediators in the country.
The mission still numbers just eight observers out of a planned initial deployment of 30.
Reports from Syria cannot be independently verified as the government has barred international journalists and rights groups.
UN Secretary-General Ban Ki-moon has given the go-ahead for the deployment of 300 ceasefire monitors to Syria from next week.
Up to six members of the Irish Defence Forces are to be made available for the mission.
The Government approved the deployment this morning.
A spokesman for the Defence Forces said Irish diplomats in New York will communicate the Government’s offer to the UN.
The spokesman said the UN will then inform the Government about the number of officers and the seniority required.
The Defence Forces expects it will several weeks before the observers join the mission.
Mr Ban has insisted that the government of President Bashar al-Assad ensures the protection of the unarmed observers and allow them to travel freely throughout the country.
The Syrian Observatory for Human Rights said Government troops repeatedly attacked the Arbaeen neighbourhood of the central city of Hama and its environs with light and heavy machine guns.
Video footage posted online by activists showed mortar rounds hitting the area, with plumes of smoke rising to the sky.
The UN observers visited several rebel suburbs near the capital and were met by thousands of protesters demanding the collapse of the regime.
Monitors also visited the town of Zabadani, where regime forces and rebel fighters have clashed repeatedly in past months.
Activists' videos showed monitors passing by army tanks posted in the streets, despite a call within UN-Arab League envoy Kofi Annan's six-point peace plan for the withdrawal of armour from residential areas.
However, Deputy Foreign Minister Faisal Meqdad stressed his government's "total commitment to respecting the Annan plan," adding that the "armed terrorist groups" - a reference to the rebels - had not yet accepted it.
Arab League chief Nabil al-Arabi called for a political process to resolve the crisis.
"A political solution cannot be considered separately from today's goal of achieving a ceasefire," Mr Arabi told reporters.
Two members of the advance team of observers set up base on Sunday in Homs.
State news agency SANA said the military observers sent to monitor the 12 April ceasefire toured the battered city's Al-Waer neighbourhood.
Activists have been sceptical of the UN mission, saying the regime was simply buying time and was not committed to the ceasefire plan.
Despite a lull in the fighting in regions visited by the observers, the violence has continued unabated in other areas, including Damascus, Hama and Daraa provinces, they say.