The United Nations said its monitors in Syria were fired upon Thursday and prevented from accessing the site of a new massacre that has ramped up fears of a drift toward all-out civil war.

UN chief Ban Ki-moon said President Bashar al-Assad has "lost all legitimacy," but Western powers appeared to have little idea how to end the violence as key Syrian ally Russia continued to oppose stronger action.

The Syrian Observatory for Human Rights said at least 55 people were killed in Wednesday's assault on Al-Kubeir, a small Sunni farming enclave surrounded by Alawite villages in the central province of Hama.

Pro-regime militiamen swept through the farmlands, slaughtering women and children, activists said. The Syrian opposition reacted by urging more armed rebellion to bring down Assad's brutal and defiant regime.

The Al-Kubeir incident comes after at least 108 people were killed in a May 25-26 massacre near the central town of Houla, most of them women and children who were summarily executed.

Addressing a special session of the UN General Assembly hours after reports emerged of the slaughter in Al-Kubeir, Ban condemned the assault as "shocking and sickening" and blamed Assad.

"The trail of blood leads back to those responsible," he said. "Any regime or leader that tolerates such killing of innocents has lost its fundamental humanity."

The United Nations said its four-vehicle convoy was hit by small arms fire in the nearby protest hub of Hama while en route to Al-Kubeir. A vehicle was damaged but the observers were unhurt.

"The patrol was forced to withdraw to a nearby government checkpoint," said UN spokesman Farhan Haq. "The monitors were not able to enter Al-Kubeir today. They will try again tomorrow."

Ban told a later UN Security Council meeting that heavy weapons, armor-piercing bullets and surveillance drones had all been used against UN observers in Syria to hamper their efforts to monitor the worsening conflict.

-- Video shows victims --

Regime forces are accused of bombarding the tiny settlement of Al-Kubeir before pro-militia thugs went on an afternoon killing spree, hacking, stabbing and shooting residents.

A resident from a nearby village told AFP the charred bodies of women and children still lay across Al-Kubeir on Thursday.

"I saw something you cannot imagine. It was a horrifying massacre... people were executed and burned. Bodies of young men were taken away," said Laith, who gave only his first name for fear of retribution.

"There are 49 confirmed and identified victims in Al-Kubeir, the majority of them from the Al-Yateem family," said Rami Abdel Rahman, head of the Britain-based Observatory.

"Among the dead are 18 women and children," he said, adding that six other people were also killed on Wednesday in a village near Al-Kubeir.

A video posted on YouTube showed bodies of several children, including babies, wrapped in blankets and white plastic body bags. Some were charred beyond recognition. The Syrian government denied its authenticity.

Damascus denied responsibility and, as it has done repeatedly in the past, pointed the finger at "terrorists" backed by foreign forces.

"A terrorist group committed a heinous crime in the Hama region which claimed nine victims. The reports by the media are contributing to spilling the blood of Syrians," state media said.

The White House condemned the "outrageous targeted killings of civilians" and said that, coupled with the regime's refusal to let UN observers verify the reports, it was an "affront to human dignity and justice."

The United States is pushing for a full transfer of power in Syria but is coming up against strong opposition from Russia and China, whose leaders fear foreign intervention could set dangerous precedents for their own countries.

-- Fears of 'all-out civil war' --

Expressing horror at the latest massacre, UN-Arab League envoy Kofi Annan told the major powers that it was time to threaten "consequences" if Assad does not act to halt the strife.

The international envoy, who secured Assad's agreement to a six-point peace plan, grimly told the UN General Assembly: "I must be frank and confirm that the plan is not being implemented."

He warned that without change in Syria, "the future is likely to be one of brutal repression, massacres, sectarian violence and even all-out civil war."

Annan briefed the UN Security Council later in the day and, according to diplomats attending the closed-door meeting, said the Syria crisis will "spiral out of control" unless substantial pressure is put on Assad.

US Secretary of State Hillary Clinton called the latest atrocity "simply unconscionable," and said a solution to the crisis required a ceasefire, a transfer of power and the formation of a representative interim government.

Russia and China have vetoed two Security Council resolutions against Assad's regime, but backed Annan's blueprint to end the conflict in which the Observatory says more than 13,500 people have died since March 2011.

Russian Foreign Minister Sergei Lavrov vowed there would be no UN Security Council mandate for outside intervention in Syria, indicating Moscow would use its veto to block any military action.

China reiterated through the state news agency Xinhua that it was firmly opposed to "outside armed intervention" in Syria or "any attempt to forcibly promote regime change."

As Washington desperately searches for viable interlocutors to help end the conflict, the opposition Syrian National Council meets in Istanbul to choose a new leader this weekend, with insiders saying Kurdish activist Abdel Basset Sayda has emerged as a consensus candidate.

Despite international outrage at the latest massacre, the violence was unrelenting. Activists said at least 41 people, including 23 civilians, were killed across Syria on Thursday.

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