Speaking to reporters after international mediator Kofi Annan's deputy, Jean-Marie Guehenno, gave the 15-nation council a bleak assessment of the impact of Annan's efforts to halt the violence in Syria, U.S. Ambassador to the United Nations Susan Rice said there were three ways the Syrian conflict could end.

The first would be if Syrian President Bashar-al Assad's government decided to comply with its obligations under Annan's six-point peace plan - stopping its military assaults on Syrian towns, withdrawing heavy weapons, returning troops to barracks and talking with the opposition on a "political transition."

The second option would involve the council taking action to pressure Damascus to fully comply with the Annan plan, she said.

Neither of those scenarios appear likely because Damascus has shown no interest in living up to its commitments and Russia has made clear that any Security Council moves to step up the pressure on Assad's government through sanctions are out of the question.

"In the absence of either of those two scenarios there seems to be only one other alternative, and that is indeed the worst case," Rice said, adding that it was unfortunately looking like "the most probable."

"That is that the violence escalates, the conflict spreads and intensifies," she said. "It involves countries in the region, it takes on increasingly sectarian forms, and we have a major crisis not only in Syria but in the region."

In such a case, Rice said, the Annan plan would be dead and the Syrian violence would become "a proxy conflict with

arms flowing in from all sides." "And members of this council and members of the international community are left with the option only of having to consider whether they're prepared to take actions outside of the Annan plan and the authority of this council," she said.

She did not specify what kind of "actions" she meant. The United States has led past military interventions that were not authorized by the Security Council, namely in Kosovo and Iraq.

So far the United States and its Western allies have rejected military options and said they would not arm the rebels.

German Ambassador Peter Wittig said council members expressed revulsion at the massacre at Houla last week, where more than 100 people, including many children were killed, probably by Syrian government forces and militia.

Wittig urged the council to consider a resolution that would sanction "spoilers" of the Annan plan and suggested that it think about whether the mission of unarmed observers should be expanded.

The British and French envoys also spoke of the need to intensify pressure on Assad's government.