Syria's main rebel commander urged Kofi Annan on Thursday to announce that his peace plan has failed in order to 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 al-Assad to abide by the conditions of Annan's plan.
"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 the U.S. 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.
Susan 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 Muslim country whose Alawite leader is allied to Shi'ite power 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.
U.N. 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 killings, an accusation that Damascus has denied.
U.N. 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 army commander in Israel, which seized the Golan Heights from Syria in a war 45 years ago, said the country was heading for collapse and would become a "warehouse of weapons" for Islamist militants.
Asaad said rebels had so far honoured 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 ceasefire 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.
Outrage at last Friday's massacre in the Houla region northwest of Homs led a range of Western countries to expel senior Syrian diplomats on Tuesday and to press Russia and China to allow tougher action by the U.N. Security Council.
Beijing said on Thursday more time should be given to allow implementation of the plan brokered by Annan, the joint United Nations and Arab League envoy for Syria.
"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.
Senior Arab officials pressed China to use its influence to help stop the violence in Syria.
"We hope it will redouble (its) effort to stop the machine of violence and death and to put more pressure on the Syrian government to respect its commitments under the Annan plan," Kuwaiti Foreign Minister Sheikh Sabah Khaled al-Sabah said on the sidelines of an Arab-China cooperation forum in Tunisia.
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.
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 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 steps 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, his office said. Lebanese sources said he would meet Lebanon's president in Beirut later in the day.
Major-General Robert Mood, Norwegian head of the observer mission, said on Wednesday the 13 corpses found in Assukar, 50 km (30 miles) 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 have been killed.
The unrest has spilled over several times into neighbouring 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.