United Nations Under-Secretary-General for Peacekeeping Operations Herve Ladsous has said that Syria's 15-month uprising has grown into a full-scale civil war.
He said that President Bashar al-Assad's forces are trying to recapture swathes of urban territory lost to rebels.
The International Red Cross has said that the situation was deteriorating in several parts of Syria simultaneously as fighting intensifies.
Many hundreds of people, including civilians, rebels, members of Assad's army and security forces, have been killed since a ceasefire deal was brokered.
UN observers overseeing ceasefire deal were turned back from the town of Haffeh by angry crowds throwing stones and metal rods.
As they left, three of their vehicles were fired on, although the monitors were unhurt.
Elsewhere, a UN report has said that Syrian troops have tortured children, executed them and used children as young as eight as "human shields" during military raids against rebels.
The UN have said that the Syrian government as one of the worst offenders on its annual "list of shame" of conflict countries where children are killed, tortured and forced to fight.
Human rights groups estimate that about 1,200 children have died during the 15-month uprising against President Bashar al-Assad.
"Rarely, have I seen such brutality against children as in Syria, where girls and boys are detained, tortured, executed, and used as human shields," Radhika Coomaraswamy, UN special representative for children in armed conflict, told AFP.
Government forces rounded up dozens of boys aged eight to 13 before an attack on the village of Ayn l'Arouz in Idlib province on 9 March, the report said.
Reports from Syria cannot be independently verified as state authorities have barred international journalists and rights groups.