Syrian President Bashar al-Assad today faced mounting international condemnation at the United Nations on Thursday as Syrian forces blocked U.N. monitors from investigating a fresh massacre site in a village near the city of Hama.

The standoff came as special emissary Kofi Annan acknowledged that his six-point plan for a political transition in Syria has reached a dead end, with both sides refusing to implement it. He said a reinvigorated diplomatic push would be required to avert a full-fledged civil war, and he warned for the first time that any party blocking a political transition should face unspecified “consequences.”

U.N. Secretary General Ban Ki-moon said the monitors came under small-arms fire Thursday as they tried to reach the site of the reported new massacre in Qubair, a small village in Hama province. Speaking at a U.N. General Assembly session, Ban said the incident occurred after the U.N. monitors were blocked from entering Qubair to investigate the alleged killings. Ban provided no details on who had fired at the monitors or whether there were any injuries. U.N. monitors have frequently been fired at since they arrived in Syria to monitor a fragile cease-fire.

Ban condemned the reported massacre as “an unspeakable barbarity” and called on the Syrian government to immediately implement the U.N.-backed peace plan.

The reported killing Wednesday of as many as 78 civilians, most of them women and children, added to mounting pressure on Assad, 46, who has ruled Syria since his father’s death in 2000.

“Clearly, the time has come to determine what more can be done to secure implementation of the [six-point] plan — and what other options exist to address the crisis,” Annan, a former U.N. secretary general, told the U.N. General Assembly at a special session on Syria attended by Ban and Nabil Elaraby, the head of the Arab League. “If things do not change, the future is likely to be one of brutal repression, massacres, sectarian violence, and even all-out civil war. All Syrians will lose.”

With U.N. observers blocked from reaching the site, it was not immediately possible to confirm that 78 people were killed Wednesday afternoon in Qubair, a small Sunni hamlet northwest of Hama that is surrounded by villages populated by members of Assad’s minority Alawite sect.

But activists and residents contacted in the area described circumstances similar to those in Houla, a Sunni village northwest of Homs where 108 civilians were massacred on May 25, allegedly by pro-government militiamen known as shabiha from the surrounding Alawite villages aided by the security forces.

Qubair resident Laith Hamawi said his mother and six brothers were among the victims. He said he was about half a mile away in his olive fields when he saw security forces and shabiha members converge on the village of about 150 residents from three directions.

“I was scared to move, so I hid,” he said.

For several hours, he said, he heard shooting and tank fire and saw houses burning. After the troops left, he returned home and found the bodies of his mother and four brothers, he said. The bodies of two other brothers were apparently taken away by the departing security forces, he said.

“We moved through the houses and saw the dead bodies of women, children and the elderly,” Hamawi said.

Most of those killed were from the Yateem family, and 40 were women and children, other activists said. They said government forces took at least 30 of the corpses with them and that other bodies were burned when homes were set on fire. Residents waited much of the day to show the bodies to U.N. monitors, but when they did not arrive, the villagers buried the remaining dead.

The official Syrian Arab News Agency (SANA) denied as “absolutely baseless” the reports that Assad loyalists had carried out the killings. Government forces moved into the area and killed “terrorists” after an “armed terrorist group” murdered nine women and children, the agency said.

Hamawi said his cousin was seized by the security forces at the entrance to the village when he heard the U.N. monitors were on the way. Hamawi said he later saw his cousin on the pro-government al-Dunia television station testifying that “armed gangs” had killed the victims.

Annan, the U.N. special envoy, has been seeking support for a plan to set up a new negotiating bloc — or contact group — including representatives from the United States, Russia, Iran and other regional and global powers, according to diplomats familiar with the proposal. The group would be tasked with drawing up a transitional plan, including presidential and parliamentary elections, and using their influence to persuade the rival parties to accept it.

Despite Annan’s call for unity, there remained sharp differences between, on the one hand, Western and Arab governments, which favor the imposition of sanctions on Syria, and, on the other, China and Russia, which have resisted sanctions and oppose any international effort to oust the Syrian leadership.

In Washington, White House press secretary Jay Carney issued a statement calling on “all nations to abandon support for this brutal and illegitimate regime, and to join together to support a political transition in Syria — one that upholds the promise of a future for which far too many have already died.”

“The United States strongly condemns the outrageous targeted killings of civilians,” including women and children in Qubair. He denounced Assad’s “continued abdication of responsibility for these horrific acts,” which “only further underscores the illegitimate and immoral nature of his rule.”

As the scale of the killings became clear, Secretary of State Hillary Rodham Clinton convened a meeting with European and Middle Eastern counterparts in Istanbul, where she was attending a conference on counterterrorism.

“The regime-sponsored violence that we witnessed again in Hama yesterday is simply unconscionable,” Clinton said at a joint news conference with Turkish Foreign Minister Ahmet Davutoglu. “Assad has doubled down on his brutality and duplicity, and Syria will not, cannot, be peaceful, stable or, certainly, democratic until Assad goes.”

Ban, meanwhile, presented the U.N. General Assembly with a gloomy assessment of conditions in Syria, warning that the prospects for peace were slipping away and that the “dangers of full-scale civil war are imminent and real.” He held Assad primarily responsible for the worsening violence in Syria.

“For many months, it has been evident that President Assad and his government have lost all legitimacy,” he said. The slaughter of civilians in the village of Houla two weeks ago “brought this fact into horrifying focus,” Ban said. “Men, women, even children were executed at point-blank range, some of their throats slit or skulls crushed.”

Expressing revulsion at the “shocking and sickening” reports of new massacres in Qubair and the village of Kafr Zeta, Ban said that U.N. monitors seeking to visit the site were blocked by a Syrian army checkpoint and fired upon by unspecified gunmen as they approached the village.

“A village apparently surrounded by Syrian forces, the bodies of innocent civilians lying there, they were shot, some allegedly burned or slashed with knives,” he said. “We condemn this unspeakable barbarity.”

Syria’s U.N. envoy, Bashar al-Jaafari, denied that his government was responsible for the killings in Qubair or the earlier massacre at Houla. Jaafari accused “armed groups” aligned with the opposition of committing “serious violations of human rights” in an effort to derail the peace process. He also faulted Ban for accusing Syria of responsibility for the Houla massacre before the United Nations has concluded its own investigation into the killings.

Maj. Gen. Robert Mood, the Norwegian leader of the U.N. observer mission in Syria, said some monitors trying to reach Qubair “are being stopped by civilians in the area.”

Mood also quoted residents as saying the safety of the observers could be at risk if they entered the small village, a cluster of farmhouses with a population of just a few hundred people about 12 miles northwest of Hama.

Observers are still trying to reach the village, but the U.N. mission “is concerned about the restriction imposed on its movements, as it will impede our ability to monitor, observe and report,” Mood added.

Sly reported from Beirut. Staff writer Joby Warrick in Washington and special correspondent Suzan Haidamous in Beirut contributed to this report.