BEIRUT (Reuters) - Syrian rebels said on Monday they were no longer bound by a U.N.-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 U.N. 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 spokesman 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 U.N. 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 U.N. Secretary-General Ban Ki-moon told Reuters Annan's mission "remains central" to resolving the Syrian crisis. Annan has inserted 300 U.N. 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 spokesman 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 U.N. 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 U.N. chief told Reuters. "Annan's plan remains central to the resolution of the Syrian crisis."
U.N. diplomats in New York say it is not clear whether the Security Council will renew the increasingly risky U.N. 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.
U.S. officials do not appear ready to give up on Annan's Syria mission, instead hoping that Russia, Assad's staunchest backer among major powers, can be coaxed into discussing a diplomatic solution that might lead to his departure.
U.S. Secretary of State Hillary Clinton spoke by telephone to Annan while in Stockholm on Monday, briefing him on her efforts to engage Russia on a "political transition" to remove Assad from power, a senior U.S. State Department official said.
The official, who spoke on condition of anonymity, said Clinton had invited Annan to Washington for talks on Friday and that Clinton expected to meet senior officials from Europe and the Middle East to discuss Syria on Wednesday in Istanbul.
The latest flare-up in violence can only intensify alarm that Syria is sliding deeper into a civil war envenomed by sectarian rancor between Assad's minority Alawites and the Sunni Muslim majority.
France's foreign minister said Syria's troubles must not be allowed to spread to its neighbors, such as Lebanon where 13 people were killed last week in Sunni-Alawite clashes.
"France has an attachment to Lebanon and we very strongly wish to avoid a situation where the people of Lebanon suffer anew from what is happening Syria," Laurent Fabius said after meeting his German counterpart Guido Westerwelle in Berlin.
Both said their governments were urging Russia to step up pressure on Syria to comply with Annan's plan.
Syria's uprising, inspired by Arab revolts elsewhere, began with peaceful protests, but the United Nations says Assad's forces have killed more than 10,000 people since March, 2011.
Damascus accuses armed "terrorists" of killing more than 2,600 soldiers and other members of the security forces.
Moscow has broadly backed Assad's narrative, highlighting rebel violence, criticizing sanctions and saying political decisions cannot not be imposed from outside.
China's top state newspaper, the People's Daily, said that any Western-backed military intervention would unleash even bloodier chaos, and that abandoning Annan's plan could push Syria into the "abyss" of full-scale war.
Additional reporting by Mariam Karouny in Beirut, Asma Alsharif in Jeddah, Denis Dyomkin in St Petersburg, John O'Donnell in Brussels, Chris Buckley in Beijing, Arshad Mohammed in Stockholm and Mahmoud Habboush in Dubai; Writing by Ralph Gowling; Editing by Giles Elgood