U.N. Secretary-General Ban Ki-moon called on Syria Thursday to stop its attacks, saying U.N. observers monitoring the cease-fire were not there to watch the killing of innocent people.

"We are there to record violations and to speak out so that the perpetrators of crimes may be held to account," Ban told a summit of the Alliance of Civilizations, a forum promoting understanding between the Western and Islamic worlds, days after more than 100 people were massacred in Syria's central Houla region.

"The more the international community knows," Ban said, "the more likely it is that we can advance on our most important goal: to help find a political solution, a solution that safeguards the lives and interests of all the Syrian people."

"Let me state plainly, however: The UN did not deploy in Syria just to bear witness to the slaughter of innocents," he said. "We are not there to play the role of passive observer to unspeakable atrocities."

Ban said that U.N. envoy Kofi Annan has expressed his concern that a "tipping point" may have been reached in Syria.

"The massacre of civilians of the sort seen last weekend could plunge Syria into a catastrophic civil war — a civil war from which the country would never recover," Ban said. "I demand that the government of Syria act on its commitments under the Annan peace plan. A united international community demands that the Syrian government act on its responsibilities to its people."

Ban urged nations to speak more loudly in "these difficult times, in the face of humankind's terrible capacity for inhumanity."

"We hear a great deal about the so-called 'clash of civilizations' — the supposed rift between predominantly Muslim and Western societies," he said. "That is not what is going on in Syria. There, it is the old story of a tyranny seeking to hold power. And in seeking to hold on to power, the regime threatens to exacerbate tensions among Syria's diverse people, much as we saw in the former Yugoslavia two decades ago."

Turkish Prime Minister Recep Tayyip Erdogan, meanwhile, called on the world to pay heed to the desperation of families whose children are massacred in Syria.

Erdogan said these are "our children who are massacred in Hama, Homs and Houla, as much they are the children of desperate Syrian families."

He said the world should not remain silent in the face of "oppression."

At U.N. headquarters in New York, U.S. Ambassador Susan Rice reiterated that there are three scenarios in Syria — the "desired and best" being for Syria to fully implement international envoy Kofi Annan's six-point peace plan.

But she said if this is to happen "Syria is going to have to feel much greater pressure, particularly from its partners and supporters" including Russia, and therefore the U.N. Security Council must apply that pressure.

On Wednesday, Rice said that pressure should include sanctions. She also said the worst but most probable scenario is a failure of Annan's peace plan and a spreading conflict that creates a major crisis not only in Syria but the region.

Rice told reporters Thursday that if the first two scenarios fail "the inevitable outcome of the status quo is that the situation continues to deteriorate with grave risks for the region as well as for the people of Syria."

"What we're looking for is a Syrian-led political transition that ... has to lead to the emergence of a new government that is representative of the will of the Syrian people," she said. "And we believe that that process would by definition lead to Assad's departure."

Rice said whether this is achieved by following the transition in Yemen, where the president stepped down and the vice-president took over, "or some other style, I think we are open about and prepared to discuss."