WORSENING violence in Syria forced United Nations (UN) observers to suspend operations on Saturday, in the clearest sign yet that a peace plan brokered by mediator Kofi Annan has collapsed.

Chief monitor Robert Mood said the fighting posed a threat to his unarmed observers, with one patrol fired upon four days ago, and prevented them from carrying out their mandate to oversee Mr Annan's widely ignored April 12 ceasefire. The Norwegian peacekeeper blamed both government troops and rebels for the conflict, in which President Bashar al-Assad's forces are trying to crush an increasingly well-armed insurgency which grew out of a 15-month-old wave of protests.

"There has been an intensification of armed violence across Syria over the past 10 days," Gen Mood said. "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."

Diplomats say Gen Mood is expected to brief the UN Security Council today or tomorrow on the unrest in Syria, which the UN's peacekeeping head described this week as a full-scale civil war.

The US said it was consulting with international partners on "next steps" and called on Syrian authorities to uphold commitments to Mr Annan's peace plan "including the full implementation of a ceasefire".

British Foreign Secretary William Hague condemned Mr al-Assad's government for failing to halt the killing and said the worsening instability "calls into serious question the viability of the UN mission" in Syria.

Despite their condemnation of Mr al-Assad, Washington and its western allies have shown no appetite for a Libya-style military intervention, while veto-wielding UN Security Council members Russia and China have shielded Damascus from the imposition of sanctions.

US President Barack Obama will hold talks with Russian President Vladimir Putin today on the sidelines of a Group of 20 summit in Mexico, but expectations are low for progress in breaking the deadlock on Syria.

Gen Mood said the violence posed "significant risks" to the 300 unarmed members of the UN Supervision Mission in Syria (Unsmis), who have been operating in the country since late April.

"In this high-risk situation, Unsmis is suspending its activities. UN observers will not be conducting patrols and will stay in their locations until further notice," he said. The decision would be reviewed daily.

Last Tuesday shots were fired at a car carrying UN observers who were turned away from the town of Haffeh by angry al-Assad supporters throwing stones and metal rods.

Three UN cars were also damaged last month when they were caught up in an attack that killed 21 civilians in Khan Sheikhoun.

Syria's foreign ministry said it had been informed of Gen Mood's decision on Friday evening and had told him it understood his concern for the safety of the monitors, blaming the attacks on rebels.

Since the start of the ceasefire the "armed terrorist groups" - the label Damascus gives to anti-al-Assad fighters - had escalated their "criminal activities, which have often targeted the UN observers", said a foreign ministry statement.

Many hundreds of people, including civilians, rebels and government forces, have been killed in the two months since Mr Annan's ceasefire was supposed to come into effect. But the violence increased sharply this month, with rebels formally abandoning any commitment to the ceasefire and government forces using attack helicopters and artillery to pound opposition strongholds.

The pro-opposition Syrian Observatory for Human Rights said at least 50 civilians were killed on Saturday, most of them in army shelling in Damascus.