Beirut - Syrian rebels said on Monday that they were no longer bound by a UN-backed truce because President Bashar al-Assad had failed to observe their Friday deadline to implement the ceasefire and had only attacked government forces to defend “our people”.

A Syrian opposition watchdog appeared to underline the rebel statement by saying at least 80 Syrian troops were killed in a surge of attacks at the weekend.

International mediator Kofi Annan, due to brief the United Nations Security Council and General Assembly on Thursday, urged major powers to ensure his peace plan was implemented by both sides as it was the “only option on the table”. Russia has blunted Western efforts to condemn Assad and push him from power.

The May 25 massacre of at least 108 people, nearly half of them children, in the Houla area of Homs province dealt a possibly fatal blow to Annan's proposed ceasefire, which was supposed to take effect on April 12 but never did.

“We have decided to end our commitment to this (ceasefire),” said Free Syrian Army spokesperson Major Sami al-Kurdi. “We have resumed our attacks but we are doing defensive attacks which means we are only attacking checkpoints in the cities.”

Kurdi said a UN observer mission in Syria should be turned into a “peace-enforcing mission”, or that the world should impose a no-fly zone and a buffer zone to help bring Assad down.

Such ideas have gained little traction previously with Western powers, let alone their Russian and Chinese critics.

The latest violence and a defiant speech by Assad on Sunday raised questions about how long Annan can pursue his threadbare peace plan on behalf of the United Nations and the Arab League.

But UN Secretary-General Ban Ki-moon told Reuters Annan's mission “remains central” to resolving the Syrian crisis. Annan has inserted 300 UN observers into Syria to verify the non-existent truce.

Annan himself “feels that perhaps the time has come, or is approaching, when the international community has to review... the crisis in Syria and decide what needs to be done to ensure implementation of the six-point plan”, his spokesperson Ahmad Fawzi told Reuters Television in Geneva.

“(Annan) and many others have warned of Syria descending into a bloody, protracted sectarian civil war. We may be there already,” said Fawzi.

The British-based Syrian Observatory for Human Rights said local doctors had confirmed the names of 80 government troops killed by the rebels.

Insurgents told the group they had killed more than 100 soldiers and destroyed some tanks in clashes across Syria, including Damascus and Idlib province in the northwest.

Syria's state news agency reported the burial on Monday of 30 members of government forces killed by rebels.

A Syrian troop pullback was at the top of Annan's six-point plan to halt hostilities, allow peaceful protests, supply humanitarian aid and start a political transition in a country controlled by the Assad family with an iron fist for 42 years.

“The Annan mission is essentially dead, and of course most Western powers admit that,” said Michael Stephens, researcher at the Royal United Services Institute's branch in Qatar.

“Houla changed the game completely in terms of what people were willing to accept and what they were not.”

Russia and China, wary of any Western-led military intervention in Syria, say Annan's plan is the only way forward, but have twice blocked UN Security Council resolutions which would have condemned Damascus and perhaps led to sanctions.

At a summit with EU leaders on Monday, Russian President Vladimir Putin skirted the issue in public comments.

Western powers have no appetite for Libya-style military action, but have provided no alternative to Annan's efforts.

Assad has rebuffed criticism of the carnage in his country. “When a surgeon performs an operation to treat a wound, do we say to him: 'Your hands are covered in blood'?” he asked in his speech on Sunday. “Or do we thank him for saving the patient?”

Ban, speaking on the sidelines of an Islamic Development Bank meeting in Jeddah, urged Assad's government to stop the violence immediately “in the name of humanity” and to start a political dialogue with his foes.

“We are deeply troubled by what has been going on,” the UN chief told Reuters. “Annan's plan remains central to the resolution of the Syrian crisis.”

UN diplomats in New York say it is not clear whether the Security Council will renew the increasingly risky UN observer mission to Syria when its 90-day mandate expires around July 20.

Arab League chief Nabil Elaraby has asked the Security Council to expand the size and beef up the force's mandate, but one diplomat said council members saw many risks involved in that idea, which the Syrians were anyway unlikely to accept. - Reuters