The rebels of the Free Syrian Army on Thursday gave President Bashar al-Assad until Friday noon to observe the UN-brokered plan to end bloodshed in the country.
"If the Syrian regime does not meet the deadline by Friday midday, the command of the Free Syrian Army announces that it will no longer be tied by any commitment to the Annan plan...and our duty will be...to defend civilians," a FSA statement said.
Reports indicate at least 13,000 have been killed in more than 14-months of brutal repression since a popular uprising against Assad's regime erupted in March of last year.
Parties to the conflict agreed April 12 to abide by a truce brokered by former UN secretary general Kofi Annan, the UN-Arab League's peace envoy to Syria.
However, the truce has largely been ignored, despite the deployment of nearly 300 UN observers on the ground. At least 1,000 have been killed in regime assaults on opposition strongholds since the truce deadline passed.
The FSA singled out in particular a May 25-26 massacre near the central town of Houla in which at least 108 people – including 49 children and 34 women – were murdered.
Some were killed by artillery and tank fire but most were killed "execution style," according to a UN report. UN investigators cited witnesses who said Assad-allied Shahiba militiamen were behind the massacre, and noted the presence of artillery shells at the site.
"After the barbarous massacre of women and children at Houla...we announce that there is no more justification for us to unilaterally respect the truce because [President Bashar al-Assad] has buried Annan's plan," the FSA statement said.
The FSA said it would announce in the coming days "a series of decisive and courageous decisions for the next phase" of their struggle against Assad's forces.
The rebels demanded that the government adhere to all six points of the Annan plan: an immediate ceasefire; ending all forms of violence; removing tanks and armored vehicles from civilian areas; humanitarian access to all regions; freeing all political prisoners; and media access nationwide.
A second massacre of at least 41 people – including 8 children – in Hama occurred Monday as Annan flew into Syria to plead with Assad to honor his UN-backed peace plan.
However, bloodshed continued in Syria as Annan left Damascus empty handed on Wednesday.
UN observer force commander Maj. Gen. Robert Mood reported that 13 bodies of people killed execution-style had been found in the eastern town of Assukar on Wednesday.
Mood described the Assakur killings as an "appalling and inexcusable act."
Such acts, however, have become almost commonplace in Syria – with reports of kidnapping, rape, torture, and mass executions by regime forces and allied militiamen emerging on a weekly basis.
The FSA ultimatum came on the heels of a warning from US ambassador to the United Nations Susan Rice warned that prospects for a political solution in Syria are now "almost non-existent."
While Rice said only that the Security Council must discuss new action against Damascus, it has also been reported officials in Washington are on the verge of arming Syria's rebels.
The FSA is comprised primarily of 30,000 defectors from the Syrian army who are armed and equipped as light infantry. Several attendant factions with suspected terror ties also exist.
Syria's rebels have mounted a deadly campaign of guerilla hit-and-run raids on ambushes on Assad's forces, but are poorly organized and equipped.
However, US officials are said to be reticent to arm Syria's rebels unless a vetting program is in place to ensure the arms end up in the right hands – which would necessitate deeper US involvement.