In a statement Saturday, major general Robert Mood, head of the UN mission to Syria, halted operations “until further notice”.
It comes amid claim that Syrian government forces has recommenced shelling in the capital Damascus, killing 12 people, according to opposition figures.
The past two weeks have seen a worrying escalation in violence in the country. A massacre in the town of Houla on May 25 resulted in the deaths of more than 100 people, many of them women and children.
That attack resulted in a series of stern warnings against the regime of President Bashar al-Assad and renewed calls for his removal from power.
But a refusal by Russia to back international pressure on Assad – amid allegations that Moscow continues to arm the strongman’s armies – has resulted in an impasse.
On Friday Mood warned that he may have to pull his 300 UN observers out of Syria unless the situation improved.
He accused both sides in the near-civil war of “willingly” intensifying the fighting, causing losses on both sides and putting unarmed UN monitors at “significant risks”.
Last week shots were fired at a car carrying international monitors after they were turned away from the town of Haffeh by angry Assad supporters who threw stones and metal rods at their convoy
On Saturday, Mood carried out his earlier threat and suspended the UN mission.
“The observers will not be conducting patrols and will stay in their locations until further notice,” he announced in a statement.
He said the uptick in violence was stopping UN personnel from carrying out their mandate to observe an April 12 ceasefire deal. That agreement has long since fallen redundant, given the continuation of killings.
“This suspension will be reviewed on a daily basis. Operations will resume when we see the situation fit for us to carry out our mandated activities,” Mood said.
The move marks yet another sign that the peace plan brokered by former UN secretary general Kofi Annan is nearing irrelevance. The fear is now that without international monitors, conditions could worsen still as Syria disintegrates into civil war.