* Annan plan is "only option on the table", spokesman says
* Peace envoy to brief Security Council on Thursday
* Annan to meet Hillary Clinton in Washington on Friday (adds Reuters TV interview, fresh quotes)
GENEVA, June 4 (Reuters) - Major powers must ensure that the peace plan for Syria is implemented by both sides, but for now international mediator Kofi Annan does not favour expanding the ceasefire monitoring mission, his spokesman said on Monday.
Annan, who is to brief the U.N. Security Council and General Assembly in New York on Thursday, will continue to press his six-point plan as "the only option on the table", spokesman Ahmad Fawzi said.
"He feels that perhaps the time has come, or is approaching, when the international community has to review the situation, the crisis in Syria, and decide what needs to be done to ensure implementation of the six-point plan," Fawzi told Reuters Television in Geneva.
Some 300 United Nations observers have been deployed in Syria to verify an April 12 truce brokered by the former U.N. Secretary-General which has yet to take hold.
The May 25 massacre of at least 108 people, nearly half of them children, in the Houla area of Homs province may have dealt a fatal blow to the ceasefire.
Fawzi called it a "turning point in the crisis".
Syrian rebels are no longer committed to a U.N.-backed peace plan that has failed to end the violence and have launched attacks on government forces to "defend our people", a spokesman for the rebel military council said on Monday.
"Many people like you have questioned whether the six-point plan has failed - whether it is the end, whether it is dead. They've written the obituary already. But we will continue to pursue the plan because it is the only option on the table at the moment," Fawzi said.
"By implementing the plan in full and simultaneously all the six points, Mr. Annan believes that we can bring peace to Syria," he said.
Asked whether the country already was engulfed in civil war, Fawzi replied: "The Special Envoy Mr. Annan and many others have warned of Syria descending into a bloody, protracted sectarian civil war. We may be there already.
"We hope for the sake of the Syrian people we're not there yet. But this is certainly a very unfortunate and bloody scenario that we are witnessing," he said.
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 U.N.-backed ceasefire.
Annan, U.N.-Arab League special envoy for Syria, returned over the weekend to his Geneva office from talks with Assad in Damascus, as well as talks in Lebanon, Jordan and Doha, Qatar where he addressed an Arab League ministerial meeting.
He will hold talks on Friday with U.S. Secretary of State Hillary Clinton in Washington, Fawzi said.
(Reporting by Stephanie Nebehay, Cecile Mantovani and Vincent Fribault; Editing by Michael Roddy)