BEIRUT - Escalating violence in Syria forced United Nations observers to suspend operations on Saturday, in the clearest sign yet that a peace plan brokered by international mediator Kofi Annan has collapsed.

Chief monitor General Robert Mood said the fighting posed a threat to his unarmed observers, one of whose patrols was fired upon four days ago, and prevented them from carrying out their mandate to oversee Annan's widely-ignored April 12 ceasefire.

The Norwegian peacekeeper blamed both government troops and rebels for the relentless conflict, in which Syrian President Bashar 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," 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 Mood is expected to brief the United Nations Security Council on Monday or Tuesday about the unrest in Syria, which the head of UN peacekeeping said this week was in the throes of full-scale civil war.

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

Despite their condemnation of 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 U.N. sanctions.

US President Barack Obama will hold talks with Russian President Vladimir Putin on Monday on the sidelines of a G20 summit in Mexico, but expectations are low for any progress to break their deadlock on Syria.

Mood said the violence posed "significant risks" to the 300 unarmed members of the UN Supervision Mission in Syria (UNSMIS), who have been operating there 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," Mood said, noting the decision would be reviewed on a daily basis.

Last Tuesday shots were fired at a car carrying UN observers who were turned away from the town of Haffeh by angry Assad supporters throwing stones and metal rods at their convoy. Three UN cars were also damaged in May when they were caught up in an attack that killed 21 civilians in Khan Sheikhoun.

Syrian government accuses rebels of targeting UN observers

Syria's Foreign Ministry said it had been informed of Mood's decision on Friday evening and told him it understood his concern for the safety of the monitors, blaming the attacks on rebels fighting government forces.

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

Many hundreds of people, including civilians, rebels and government forces, have been killed in the two months since Annan's ceasefire deal was supposed to come into effect.

But the violence has increased sharply this month, with rebels formally abandoning any commitment to Annan's ceasefire and government forces using attack helicopters and artillery to pound opposition strongholds into submission.

More than 50 killed in violence on Saturday

The pro-opposition Syrian Observatory for Human Rights said at least 28 civilians were killed on Saturday, most of them in army shelling on the eastern suburbs of Damascus.

In a sign that government forces were also suffering heavy losses, SANA reported on Saturday military funerals for 24 soldiers and members of security forces.

Syria restricts access to international media, making it hard to verify accounts by activists and authorities.

Activists also reported gunfire in the Mezze neighborhood of the capital, and bombardment of rebel strongholds in the central city of Homs which they said killed five people.

"There has been heavy shelling in Homs since early morning," a local activist who declined to be named said by Skype.

"Since 4 a.m. (0100 GMT) there was mortar and artillery shelling of Khalidiya, Old Homs, Jouret al-Shiyah and Qusur districts," he said

The British-based Observatory, which monitors violence in Syria through a network of local sources, said 1,000 families were surrounded in the Homs districts under fire from Assad's forces on Saturday.

Dozens of wounded people were in danger because of lack of medical equipment, it said.

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