BEIRUT, June 16 (Reuters) - United Nations observers monitoring the turmoil in Syria suspended operations on Saturday in response to escalating violence which threatens to kill off a tattered peace plan brokered by international mediator Kofi Annan.

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.

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

"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 head of U.N. peacekeeping said this week was now in the throes of full-scale civil war.

The five veto-wielding Security Council members are expected to have discussions on the sidelines of a G20 summit in Mexico, which starts on Monday, aimed at breaking deadlock on the issue of international sanctions against 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 U.N. observers who were turned away from the town of Haffeh by angry Assad supporters throwing stones and metal rods at their convoy. Three U.N. cars were also damaged in May 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 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 U.N. observers", a foreign ministry statement cited by state news agency SANA said.

HOMS BOMBARDED

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.

The pro-opposition Syrian Observatory for Human Rights said at least 22 civilians were killed on Saturday, most of them in army shelling on the eastern suburbs of Damascus.

Activists also reported gunfire in the Mezze neighbourhood 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

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

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.

France said on Friday night it was seriously concerned about what it said were reports of an imminent large-scale operation against Homs.

"The bloody repression led by Syrian authorities, which is intolerable and has caused tens of deaths in recent days, must come to an end," a Foreign Ministry spokeswoman said.

"Bashar Al Assad's regime continues to violate commitments under the Annan plan and threatens international peace and security. Sooner or later, the Security Council will have to reckon with the consequences."

The United Nations says Syrian forces have killed 10,000 people in a crackdown on protest against Assad's rule which broke out in March last year, inspired by uprisings across the Arab world which have toppled four autocratic leaders.

Syrian authorities blame the violence on foreign-backed Islamists who they say have killed at least 2,600 police and soldiers. (Additional reporting by Nicholas Vinocur in Paris and Louis Charbonneau at the United Nations; Editing by Angus MacSwan)