The United Nations says its unarmed monitors in Syria were shot at and blocked from investigating the site of a newly reported mass killing, fueling international condemnation of President Bashar al-Assad's government.

Secretary-General Ban Ki-moon told the U.N. General Assembly Wednesday that international observers were denied access to the village of Mazraat al-Qubeir in central Hama province and “were shot at with small arms” while trying to get there.

While Mr. Ban did not directly blame Syrian forces for the latest reported atrocity, he harshly criticized what appeared to be a government effort to block monitors from visiting the site of Wednesday's massacre in which at least 78 people were killed, half of them women and children, including 35 members of one family. Some were stabbed and burned.

Kieran Dwyer, spokesman for the U.N. peacekeeping department, said the observers were forced to turn back and were not injured, although one vehicle was hit and slightly damaged.

International envoy Kofi Annan, whose peace plan brokered in April has not been implemented, warned against allowing “mass killings to become part of everyday reality in Syria.”

Annan urged the divided U.N. Security Council and the rest of the international community to unite and act immediately to intensify pressure, especially on Mr. Assad's government. If things do not change, he warned, Syria will likely face a future of “brutal repression, massacres, sectarian violence and even all-out civil war” in which “all Syrians will lose.”

U.N. diplomats said Annan proposed that world powers and key regional players, including Iran, come up with a new strategy to end the 15-month conflict at a closed meeting of the Security Council on Thursday. Annan is scheduled to meet with U.S. Secretary of State Hillary Clinton Friday in Washington.

If confirmed, the Qubeir massacre would be the fourth such mass slaying of civilians in Syria in the last two weeks.

The White House Thursday accused the Syrian government of orchestrating “the outrageous targeted killings of civilians, including women and children…as reported by multiple credible sources.”

Thousands of Syrians have died in attacks and clashes since the cease-fire was put in place, and the presence of hundreds of U.N. observers has not been able to stem the violence.

Speaking Thursday in Istanbul, Clinton said Mr. Assad has “doubled down on his brutality and duplicity,” and that the time has come for a post-Assad Syria.

“We have to unite the international community behind a plan that is achievable and keeps faith with those inside Syria who are protesting and demonstrating, suffering and dying for their universal human rights.”

Even as she criticized the Syrian leader, China – one of Mr. Assad's leading allies along with Russia – reiterated its opposition to any effort to oust him outside the existing Annan plan to end the fighting, seeming to limit prospects for any breakthrough.

Bob Moog, of North Carolina State University, said Russian interests will need to be taken into consideration for the peace plan to take hold.

“It's going to take a realization on the part of the Russians that they are essentially on the losing side, that the Assad regime is not going to survive a civil war in Syria, and that they are going to lose everything if they continue to hold out and fail to cooperate with much of the rest of the world.”

Moog also says expectations are low for what the U.N. can do to help the Syrian situation.

“I think many people in the world, rightly or wrongly, look to the U.S. as the final arbiter in this. And if the U.N. doesn't act, then there's going to be expectations from some quarters that the U.S. should act in a unilateral, or if not unilateral, than in a multilateral matter, but outside the confines of the U.N.”

The Britain-based Syrian Observatory for Human Rights said pro-government 'shabiha' militiamen armed with guns and knives carried out the attack in Mazraat al-Qubeir after regular troops had shelled the area.

Qubeir, on the outskirts of Hama, is small and mostly Sunni Muslim. Residents said shabiha raided the settlement from two neighboring villages of Alawites, a minority Muslim sect to which the Assad family and high-ranking members of the military and security apparatus belong to.

The exiled opposition Syrian National Council called on the Free Syrian Army and other armed opposition groups to respond by escalating their resistance.

British Prime Minister David Cameron called the reported attack “brutal and sickening.” He said that if true, the international community must do more to condemn “absolutely” the government of President Assad.

The Syrian government blamed a terrorist group for the violence.

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