The chief UN observer in Syria has announced that all operations have been suspended "until further notice" citing an escalation in violence over the past 10 days
The UN observer mission in Syria - UNSMIS - consisted of some 300 unarmed monitors who were overseeing the implementation of a ceasefire after months of fighting bewteen Syrian government and opposition forces.
In Syria, the head of the UN mission put the blame on both government and rebel forces.
From the Middle East, our International Correspondent John Irvine reports.
Major General Robert Mood, the head of the mission, said in an on-camera statement:
UNSMIS was formed on April 21st by a resolution in the UN Security Council. It was tasked with observing whether both sides in the conflict adopted the six-point plan drawn up by the UN envoy to Syria Kofi Annan.
Its work has included reporting on a number of massacres of civilians, including those in Houla near Homs and Haffa in Latakia Province.
Video showed that several of the vehicles used by the observers had bullet holes, broken windows and scratches.
Many hundreds of people, including civilians, rebels and government forces, have been killed in the two months since Kofi Annan's ceasefire deal was supposed to come into effect.
Last week shots were fired at a car carrying UN observers after trying to investigate alleged massacres in the towns of Qubair and Maarzaf.
They were turned away from the town of Haffa by angry Assad supporters who threw stones and metal rods at their convoy, a spokeswoman for the monitors said.
The White House has said it is consulting with its international partners over the Syrian crisis, after intensifying violence forced UN peace monitors to suspend operations there.
British Foreign Secretary William Hague has called on the international community to take urgent action over Syria after United Nations observers suspended activities in the state following days of escalating violence.
Prime Minister David Cameron will discuss the deteriorating situation with world leaders at the G20 summit in Mexico next week, the government confirmed.
Mr Hague condemned president Bashar Assad's regime in the "strongest terms" and warned the UN Security Council will now be "considering its options".
Major General Robert Mood, head of the UN Stabilisation Mission in Syria, said the continued bloodshed was posing significant risks to the observers.
Mr Hague said: "I regret that it has been necessary for the UN Mission to decide to suspend patrols and to restrict staff movements due to escalating violence in Syria.
"This underlines the extent of the deterioration of security and stability in Syria, and calls into serious question the viability of the UN Mission.