It documents more than a dozen incidents involving at least 101 victims since late 2011, many of them in March 2012, in which Syrians were killed in cold blood by security forces and pro-government militiamen.

  "In a desperate attempt to crush the uprising, Syrian forces have executed people in cold blood, civilians and opposition fighters alike," said Ole Solvang, emergencies researcher at Human Rights Watch. 

"They are doing it in broad daylight and in front of witnesses, evidently not concerned about any accountability for their crimes."

  Human Rights Watch said the U.N. Security Council should ensure that any U.N. mission mandated to supervise the six-point peace plan brokered by the U.N.-Arab League envoy Kofi Annan "would be in a position to document such crimes." 

But with less than 24 hours to go before the first deadline for implementation of the ceasefire plan, there was little sign of any let-up in the fighting in Syria that would permit the deployment of a small, unarmed UN observer mission.