U.S. Secretary of State Hillary Clinton and U.N.-Arab League envoy Kofi Annan led increased diplomatic pressure on Syria Thursday following reports of another massacre that killed dozens of Syrians.

Addressing the U.N. General Assembly, Mr. Annan said the international community must take its Syria response “to another level.”

Mr. Annan admitted that more than a month after the Syrian government and opposition forces agreed to his cease-fire plan, “it is not being implemented.” The envoy said it is time to see “what other options exist to end the violence.”

“The violence is getting worse. The abuses are continuing. The country is becoming more polarized and more radicalized. And Syria's immediate neighbors are increasingly worried about the threat of spillover.”

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.

The Obama administration again called for regime change in Syria.

Speaking Thursday in Istanbul after reports that 78 people were killed in Syria in a Wednesday massacre, Clinton said that Syrian President Bashar al- Assad has “doubled down on his brutality and duplicity,” and that the time has come for the international community to plan 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.”

Clinton also said it was important to give Mr. Annan's peace plan “the last support” the international community can “muster,” so that others in the Security Council can come to the “frame of mind” to see that action beyond the plan is needed.

Mr. Annan is due to brief the U.N. Security Council later Thursday. Diplomats said he would present a proposal to rescue his failing peace plan.

They said Mr. Annan's new proposal would establish a “contact group” for Syria that would include the five permanent members of the Security Council and key regional players with influence on Damascus or the opposition, such as Saudi Arabia, Qatar, Turkey and Iran.

The group would attempt to map out a “political transition” leading to President Assad's departure and the holding of free elections. Mr. Annan is scheduled to meet with Secretary Clinton Friday in Washington.

Bob Moog, the chair of the Political Science Department at North Carolina State University, says that 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.”

Syrian Ambassador Bashar Ja'afari told the U.N. General Assembly Thursday that “the doors of Syria are open” to “national and international dialogue of reform.” He said his government remains committed to Mr. Annan's six-point peace plan.

The diplomatic flurry came as Syrian activists accused pro-government militia and security forces of carrying out a massacre in the central province of Hama.

The activists said dozens of people, including women and children, were killed on Wednesday in the village of Mazraat al-Qubeir. They said some of the victims were stabbed to death and at least 12 bodies were burned.

U.N. observer mission chief Robert Mood said observers are trying to verify the reports but have not been unable to reach the village. He said Syrian Army checkpoints and civilians have stopped the traveling observers, and residents have sent information that it is unsafe for the U.N. monitors to enter the village.

The U.N. reported Thursday that bullets hit vehicles containing U.N. observers. The U.N. did not release details, but said there were no injuries.

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

U.N. Secretary-General Ban Ki-moon, speaking at the start of a special U.N. General Assembly session, condemned the reported massacre as “an unspeakable barbarity.”

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.