UNITED NATIONS - The US envoy to the United Nations has warned that UN Security Council members may have to act alone to end the violence in Syria if the crisis worsens and the Council remains divided.

US Ambassador Susan Rice's remarks marked a rhetorical escalation after a massacre last week blamed on a militia loyal to Syrian President Bashar al-Assad dealt a major blow to an already fragile UN peace plan.

Speaking to reporters on Wednesday, Rice said if the Syrian government did not adhere to special envoy Kofi Annan's plan, then the UN Security Council had to "assume its responsibilities" and step up pressure on Damascus.

In the absence of either scenario, and if the violence continued to worsen, "then 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."

Urging Security Council penalties on Syria, she sketched out an alternative situation in which violence escalates, spreads to other countries in the region and finally breaks into a sectarian proxy war with arms flowing in from all sides. In that case, she said, Security Council members would be left to consider taking action outside the UN, a scenario she said the US and allies have sought to avoid.

Rice's remarks appeared to signal that Washington and its allies would consider whether to act alone should Russia and China continue to block tough action against Syria over its brutal 14-month crackdown on protests.

"The decision rests, in the first instance, with the Syrian government, whether it will fulfill its commitments. And if it does, then the opposition has an obligation to reciprocate," Rice said.

"If it doesn't, this council has a responsibility to act and act swiftly and surely. And if we don't, then we are all resigning ourselves to a third scenario, which we still hope to avoid."

More than 100 people were killed in the central town of Houla on May 25-26, including 49 children and 34 women, according to UN military observers who visited the site.

Some were blown to bits by artillery and tank fire but most were summarily executed, according to the UN High Commissioner for Human Rights. UN officials suspect the massacre was carried out by a pro-government militia.

In a coordinated move on Tuesday, the United States and several Western powers expelled Syrian diplomats over the massacre and warned Assad that time was running out for the battered UN-brokered peace plan.

More than 13,000 people have been killed, mostly civilians, since an uprising erupted in March 2011 against the Assad family's 40-year rule, according to the Syrian Observatory for Human Rights.