Syrian activists threatened a "volcano of rage" Friday over the killing of civilians by government forces as a deadline set by rebel fighters passed for Damascus to honor a UN-backed ceasefire. The United Nations said Syrian forces and pro-government militia accused of committing a massacre in Houla could face prosecution for crimes against humanity as rights experts said Syrian authorities had directly ordered torture. UN High Commissioner for Human Rights Navi Pillay called again for the Security Council to refer Syria to the prosecutor of the International Criminal Court (ICC) and for world powers to help implement Kofi Annan's peace plan to end the violence. In a speech read out on her behalf to an emergency session of the UN Human Rights Council, she cited allegations that the Syrian military unleashed a barrage of heavy weapons on the town of Houla a week ago and that shabbiha groups killed dozens of the 106 victims, including women and children. "These acts may amount to crimes against humanity and other international crimes and may be indicative of a pattern of widespread or systematic attacks against civilian populations that have been perpetrated with impunity," she said. Pillay, a former war crimes judge, added: "I reiterate that those who order, assist or fail to stop attacks on civilians are individually criminally liable for their actions." On the first Friday since the killings in Houla, opposition activists called on Syrians to rise up across the country in honor of the 49 children who were among the 108 dead counted by the UN mission. "A new volcano of rage is exploding thanks to them," protest organizers said on their Syrian Revolution 2011 Facebook page, which has been a major engine of the 15-month uprising against Assad's rule. US Ambassador Eileen Chamberlain Donahoe said there was no doubt that the regime of President Bashar Assad was responsible for the killing. "There needs to be justice and accountability for those that committed these atrocities," she told the council. A draft resolution proposed by Qatar, Turkey and the United States condemns the killings in Houla and states that "those responsible for serious violations of human rights must be held accountable," but doesn't suggest how. European diplomats want the resolution to include a call for the UN Security Council in New York to consider referring the massacre to the International Criminal Court. This is something the rights council cannot do on its own. And since Syria isn't a member of the ICC, under international law only the Security Council can refer it to the Hague-based tribunal. "Mostly we are pressing for some stronger language on accountability," said Maria Ulff Moeller, a Danish diplomat whose country currently holds the rotating presidency of the European Union. "We can encourage the Security Council to refer the situation to the ICC, and it's something we are pushing for." Human rights groups backed the EU position. "At this stage what we need is a strong resolution requesting ICC referral," said Juliette de Rivero, a spokeswoman for the group Human Rights Watch. But other nations who favor sharp language critical of Syria are questioning whether the rights body should invoke the war crimes tribunal. __