Syrian rebels have given President Bashar al-Assad's regime an ultimatum, warning that if Damascus does not comply with the UN-backed cease-fire in 48 hours, they will no longer be bound the agreement.

Colonel Kassim Saadeddine of the Free Syrian Army, a rebel group made up of deserters from the regular army, announced on Wednesday that the Assad regime had until 0900 GMT on Friday to comply with UN-Arab League Envoy Kofi Annan's peace plan.

The plan calls for an immediate cease-fire, withdrawal of Syrian troops and heavy weapons from cities, the release of detainees and free access for journalist s and aid groups in the country. Although the cease-fire went into effect on April 12, it has been violated on a near daily basis by both the government and rebels since then.

Saadeddine said that if Assad did not meet the Friday deadline, then Free Syrian Army would be "free from any commitment, and we will defend and protect the civilians, their villages and their cities."

'Appalling and inexcusable'

The rebel group's ultimatum came after UN military observers reported that 13 bodies had been discovered on Wednesday in Assukar, about 50 kilometers east of Deir al-Zor. Some of the corpses had bound hands and bullet wounds to their heads.

Major General Robert Mood, the Norwegian head of the UN observer mission, called the killings an "appalling and inexcusable act."

The killings are the latest atrocity to emerge from the Syrian conflict. On May 25, some 108 people were killed near the town of Houla. Forty-nine of the dead were children and 34 were women. UN peacekeeping chief Herve Ladsous said in New York on Tuesday that the Syrian government and militiamen from Assad's Alawite religious group were probably behind the massacre.

The Syrian government claims that terrorists are responsible for the deaths in Houla.

'Worst case scenario'

US ambassador to the UN, Susan Rice, suggested on Wednesday that the Syrian conflict had reached a crossroads. After a dismal briefing by Annan's deputy, Rice told reporters in New York that Assad was not likely to comply with the peace plan, and the UN Security Council did not seem prepared to take action.

"In the absence of either of those two scenarios there seems to be only one other alternative and that is indeed the worst case," Rice said, adding that it seemed "the most probable."

"The violence escalates, the conflict spreads and intensifies," the US ambassador said. "It involves countries in the region, it takes on increasingly sectarian forms, and we have a major crisis not only in Syria but the region."

"Members of this council and members of the international community are left with the option only of having to consider whether they're prepared to take act ions outside of the Annan plan and the authority of the council," Rice said.

French President Francois Hollande had said on Tuesday that a military intervention could be considered if the Security Council mandated it.

slk/av (Reuters, dpa)