BEIRUT – U.N. observers came under fire Thursday as they tried to reach the site of the latest reported mass killing in Syria — about 80 people, including women and children who were shot or stabbed. The deaths added urgency to diplomatic efforts to end the escalating bloodshed.

As reports emerged of what would be the fourth such mass slaying of civilians in Syria in the past two weeks, the United States condemned President Bashar Assad, saying he has “doubled down on his brutality and duplicity.”

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.”

“If things do not change, the future is likely to be one of brutal repression, massacres, sectarian violence and even all-out civil war,” Annan told the U.N. General Assembly in New York. “All Syrians will lose.”

U.N. diplomats said Annan was proposing 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 that took place Thursday.

Speaking to reporters after the meeting, Annan highlighted the urgency of taking action to diffuse the situation.

Standing alongside Annan and League of Arab States Secretary General Nabil Elaraby, U.N. Secretary-General Ban Ki-moon echoed the sense of urgency.

“The three of us agree: Syria can quickly go from a tipping point to a breaking point. The danger of full-scale civil war is imminent and real, with catastrophic consequences for Syria and the region,” Ban warned.

Any proposal to resolve the situation, however, must be acceptable to Russia and China, which have protected Syria from past U.N. sanctions, as well as the U.S. and its European allies.

The latest violence centered on Mazraat al-Qubair, a small farming community of 160 people, mostly Bedouins, in central Hama province. A resident said troops shelled the area for five hours Wednesday before government-aligned militiamen known as “shabiha” entered the area that is known to shelter army defectors, “killing and hacking everyone they could find.”

Leith Al-Hamwy told The Associated Press by telephone that he survived by hiding in an olive grove about 800 yards from the farms as the killings took place. But he said his mother and six siblings, the youngest 10-year-old twins, did not.

“When I came out of hiding and went inside the houses, I saw bodies everywhere. Entire families either shot or killed with sharp sticks and knives,” he said.

Al-Hamwy’s account could not be independently confirmed or corroborated by other eyewitnesses.

Syria’s main opposition group in exile, the Syrian National Council, also said 78 people were killed in Mazraat al-Qubair when government-aligned militiamen converged on the village from neighboring pro-regime villages. Some of the dead were shot in the head, others were slain with knives, the SNC said. It said 35 of the dead were from the same family and more than half of them were women and children.

“Women and children were burned inside their homes in al-Qubair,” said Mousab Alhamadee, an activist based in Hama.

Syria denied the opposition claims as “absolutely baseless.”

The exact death toll and circumstances of the killings reported overnight in Mazraat al-Qubair were impossible to confirm.

One YouTube video purported to show the bodies of babies, children and two women wrapped in blankets and lined with frozen bottles of water to slow decomposition.

Another row of bodies lay elsewhere: a grandmother, a mother, and five siblings and two cousins, according to the video narrator.

The authenticity of the videos could not be independently verified. Attempts to reach more witnesses and residents of the area were difficult. The Syrian government keeps tight restrictions on journalists.

A government statement published on the state-run news agency SANA said “an armed terrorist group committed an appalling crime” in Mazraat al-Qubair, killing nine women and children. It said residents appealed for protection from Hama authorities, who went to the farm and stormed a hideout of the group and clashed with them.

Secretary-General Ban said U.N. observers were initially denied access to the scene in central Hama and “were shot at with small arms” while trying to get there.

The observers were forced to turn back and were not injured, although one vehicle was hit and slightly damaged, said Kieran Dwyer, spokesman for the U.N. peacekeeping department. They were not able to enter Mazraat al-Qubair, he said. It was not clear who was behind the shooting.

On May 25, more than 100 people were killed in one day in a cluster of villages known as Houla in central Homs province, many of them children and women gunned down in their homes. U.N. investigators blamed pro-government gunmen for at least some of the killings, but the Syrian regime denied responsibility and blamed rebels for the deaths.