Armed rebels have given Syria's regime until Friday noon (0900 GMT) to observe Kofi Annan's plan for ending bloodshed in Syria, warning they will take "courageous decisions" if the deadline is not met, AFP reported.

The ultimatum by the Free Syrian Army was followed by a demand from UN chief Ban Ki-moon that the regime implement Annan's six-point plan, which includes a ceasefire that should have taken effect on April 12 but has been violated daily.

US ambassador to the United Nations Susan Rice warned on Wednesday that prospects for a political solution -- part of the Annan plan -- are now "almost non-existent" and that the Security Council must discuss new action against Damascus.

A statement issued by the Free Syrian Army command inside Syria said that if the regime "does not meet the deadline by Friday midday, the command ... will no longer be tied by any commitment to the Annan plan ... and our duty will be ... to defend civilians."

Parties to the conflict, which has seen more than 13,000 people killed since an anti-regime revolt erupted in March 2011, agreed to abide by the truce that was brokered by Annan, the UN-Arab League's peace envoy to Syria.

And despite the deployment of nearly 300 UN observers, the death toll has spiralled in recent weeks as regime forces assault opposition strongholds.

The FSA singled out last weekend's massacre near the central town of Houla in which 108 people died, including 49 children and 34 women.

Some were killed by artillery and tank fire but most were summarily executed, according to the UN High Commissioner for Human Rights.

On Wednesday, the UN observer mission chief in Syria, Major General Robert Mood, disclosed a new massacre. He said 13 bodies of people killed execution-style had been found in the eastern town of Assukar, describing it as an "appalling and inexcusable act."