The United Nations suspended all activities in Syria on Saturday due to the escalating levels of violence, the head of the global body's mission said.
"There has been an intensification of armed violence across Syria over the past 10 days," said Gen. Robert Mood, head of the U.N. Supervision Mission in Syria.
"This escalation is limiting our ability to observe, verify, report as well as assist in local dialogue and stability projects -- basically impeding our ability to carry out our mandate," Mood said.
The situation, Mood said, was too high risk.
'Choking siege' leaves Syrian town in crisis
The U.N. monitors, whose number gradually rose to about 300, were sent in to ensure that both President Bashar al-Assad and opposition fighters were abiding by a six-point peace plan brokered by special envoy Kofi Annan.
A ceasefire took hold April 12 but only nominally, it turned out.
Violence has soared in recent days with reports of heavy government bombardment of town and chilling massacres of civilians.
On some occasions, the monitors themselves have come under fire.
"The lack of willingness by the parties to seek a peaceful transition, and the push towards advancing military positions is increasing the losses on both sides: innocent civilians, men women and children are being killed every day," Mood said.