BEIRUT, June 4: Syrian rebels killed at least 80 army soldiers in a surge of attacks at the weekend, an opposition watchdog said on Monday, carrying out their threat to resume fighting if President Bashar al-Assad failed to observe a UN-backed ceasefire.

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 envoy Kofi Annan's proposed ceasefire, which was supposed to take effect on April 12 but never did.

A Free Syrian Army spokesman said the rebels were no longer bound by the truce after their Friday deadline passed.

“We have decided to end our commitment to this (ceasefire),” Major Sami al-Kurdi, told Reuters from Syria, adding that rebels had begun attacking government forces to “defend our people”.

He said a UN observer mission 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 said Annan's mission “remains central” to resolving the Syrian crisis.

The Syrian Observatory for Human Rights said local doctors had confirmed the names of 80 dead government soldiers.

Insurgents told the British-based 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 the security 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.”

However, Russia and China, wary of any Western-led military intervention in Syria, say Annan's plan is the only way forward.

They have twice blocked UN Security Council resolutions which would have condemned Damascus and perhaps led to sanctions.

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