Syria’s main rebel commander urged UN-Arab League envoy Kofi Annan on Thursday to announce that his peace plan has failed and free insurgents from any commitment to a truce deal, which the United States said may collapse and trigger a wider Middle East crisis.Colonel Riad al-Asaad, who is based in Turkey, contradicted a statement by the rebels inside Syria, who issued a 48-hour ultimatum on Wednesday for President Bashar Assad to abide by the conditions of Annan’s plan.Rice outlined what she said was both a worst case and most likely scenario in which “the violence escalates, the conflict spreads and intensifies...It involves countries in the region, it takes on increasingly sectarian forms, and we have a major crisis not only in Syria but in the region.”In that case, Syria – a mainly Sunni country whose Alawite leader is allied to Shi’ite Iran – would become “a proxy conflict with arms coming in from all sides” and world powers would consider taking unilateral actions, Rice said.The rival statements from rebels inside and outside Syria showed once again how deep divisions run between Assad’s foes, who have failed to unify either political or military operations more than 14 months after Syria’s uprising first broke out.UN observers on Wednesday reported the discovery of 13 bodies bound and shot in eastern Syria, adding to the world outcry over the massacre last week of 108 men, women and children in the western town of Houla. The United Nations has said the army and pro-Assad gunmen were probably responsible for the Houla killings, an accusation that Damascus has denied.Syria said on Thursday a preliminary investigation showed that anti-government armed groups carried out the massacre with the aim of encouraging foreign military intervention against the government.Brig.-Gen. Qassem Jamal Suleiman, head of the investigation committee formed by the government, said the victims were families “who refused to oppose the government and were at odds with the armed groups.”He said that before the massacre, 600 to 800 armed men attacked posts of the security forces in the area while armed men from outside Houla murdered the families, adding that many of the victims were relatives of a member of parliament.“The aim is to bring foreign military intervention against the country in any form and way,” he told reporters at a news conference in Damascus that was aired on television.UN Secretary-General Ban Ki-moon warned on Thursday that another atrocity could pitch Syria into a devastating civil war “from which the country would never recover.”A senior Israeli army commander said Syria was heading for collapse and would become a “warehouse of weapons” for Islamist militants.Asaad said rebels had so far honored their commitments to Annan’s plan. But activists have reported frequent attacks by militants and army defectors on Assad’s forces since the April 12 cease-fire agreement.Government forces have also bombarded towns, fired on protesters and attacked rebel strongholds, killing many hundreds of people in the last seven weeks, the activists say.Beijing said on Thursday that more time should be given to allow implementation of the plan brokered by Annan.“China believes that the situation in Syria currently is certainly very complex and serious,” Foreign Ministry spokesman Liu Weimin told a daily news briefing.“But at the same time, we believe that Annan’s mediation efforts have been effective and we ought to have even more faith in him and give him more support,” he added.Beijing and Moscow have both vetoed two Security Council resolutions calling for tougher action against Damascus, while stressing hopes for a political solution brokered by Annan.Russian President Vladimir Putin flies to Berlin and Paris on Friday for talks, which European leaders may hope to use to lean on Putin to loosen Moscow’s strategic links to Assad.“What is happening in Syria is a catastrophe,” German Chancellor Angela Merkel said.Praising Russia’s “constructive cooperation” with the Security Council, Merkel added that “there have, however, been areas where we wanted to go further.”Clinton said she had told Moscow that the chances of full-blown civil war in Syria were greater if the world failed to act.But, in contrast to Libya, where NATO forces helped rebels topple Muammar Gaddafi last year, she said Syria’s sectarian divisions, splintered opposition, stronger air defenses and armed forces were all factors against armed intervention.Ban, speaking in Turkey, said Assad must respond to world opinion.“I demand that the government of Syria act on its commitments under the Annan peace plan. A united international community demands that the Syrian government act on its responsibilities to its people,” he said.Syrian state television said on Thursday that 500 prisoners who had been arrested on suspicion of involvement in the uprising had been freed, two days after Annan urged Assad to take bold and immediate steps to rescue the plan.Annan met Jordan’s King Abdullah in Amman on Thursday to discuss the regional impact of the Syrian crisis, before flying to Lebanon where he met President Michel Suleiman.His spokesman Ahmad Fawzi, responding to Asaad’s call, said it was not for Annan to declare defeat.“The Annan plan does not belong to Kofi Annan. It belongs to the parties that have accepted it and the international community that has endorsed it,” he told Reuters.“So a failure of the Annan plan would be the failure of the international community to solve this peacefully,” Fawzi said.“If anyone has a better plan, they should come up with it.”Maj.-Gen. Robert Mood, the Norwegian head of the observer mission, said on Wednesday the 13 corpses found in Assukar, 50 kilometers east of Deir al-Zor, had their hands tied behind their backs. Some had been shot in the head from close range.Mood called the latest killings an “appalling and inexcusable act” and appealed to all factions to end the cycle of violence. He did not apportion any blame but Syrian activists said the victims were army defectors killed by Assad’s forces.The United Nations says Assad’s forces have killed more than 9,000 people since the start of the uprising, inspired by protests against autocratic leaders across the Arab world.Syria blames Islamist militants for the violence and says 2,600 soldiers and police officers have been killed.The unrest has spilled over several times into neighboring Lebanon. In the latest incident, gunmen kidnapped two Lebanese farmers in the country’s north and took them across the border into Syria on Wednesday, a Lebanese security source said.“There is no deadline, but we want Kofi Annan to issue a declaration announcing the failure of this plan so that we would be free to carry out any military operation against the regime,” Asaad told Al Jazeera television.Annan’s plan has not stemmed bloodshed in Syria and Susan Rice, the US envoy to the United Nations, warned that unless the Security Council acts swiftly to pressure Syria to end its crackdown on opposition, countries may act outside of the world body.US Secretary of State Hillary Clinton on Thursday indicated that Washington continues to support the UN peacekeeping mission in Syria rather than direct intervention by the United States.“Right now, we are focused on supporting Kofi Annan,” she said during a press conference while on a trip to Denmark, referring to the former UN secretary-general who is heading the peacekeeping mission. She also said the US is continuing to reach out to the Russians, who have previously cast vetoes preventing more aggressive action in Syria.Clinton did acknowledge, though, that the US has prepared steps that it could take on its own outside the UN framework.“We plan against everything in order to be prepared in the event that action is called for,” she said.