Syria on Sunday strongly denied allegations that its forces killed more than 100 people -- including women and children -- in what activists called one of the deadliest regime attacks yet in Syria's 15-month-old uprising. U.S. National Security Council spokesman Tommy Vietor said the United States was horrified by "credible reports" of Friday's massacre in Houla, a group of villages northwest of the central city of Homs, "including stabbing and ax attacks on women and children." Gruesome video footage from the weekend showed rows of dead children lying in rows in a mosque covered in blood and deep wounds, and the U.N. counted 49 children and 34 women among the dead. The massacre sparked outrage from the U.S. and other international leaders, prompted large protests in the cities of Damascus and Aleppo, and cast fresh doubts on a month-old international peace plan put forward by U.N.-Arab League envoy Kofi Annan to end Syria's crisis. Daily violence has marred the plan since a ceasefire was supposed to begin April 12; the U.N. put the death toll weeks ago at more than 9,000, and hundreds more have been killed since then. "Nobody can see these images and not react," said Rami Khouri of the American University of Beirut in Lebanon. "The problem is no one has figured out an effective way to get involved and bring this conflict to an end."