An Israeli official said Israel, which controls the Golan Heights, would not intervene in the situation.
"It's happening in Syria. We are following it very closely," the official said." We can't and won't interfere in the events on the other side of the border. We have offered UNDOF (the United Nations Disengagement Observer Force) any kind of assistance they might require and we hope this ends quickly with no harm to anyone."
Earlier this week, al-Khatib posted on the rebels' Facebook page a letter to U.N. Secretary-General Ban Ki-moon, the leaders of the Organization of Islamic Cooperation and the League of Arab States.
"What is happening (is a) genocide for the Syrian people with the world watching and listening (and) will lead to the gravest consequences," he wrote.
"The blood of the people of Syria will be a curse on the whole world if there" is "no effective action," it said.
There has been "hardly a Syrian village spared from the regime bombing," the letter said.
"This might be the last message to you," it warns. "I call on you all to bear your international responsibilities before God and the people."