U.N. monitors came under fire in Syria on Thursday while trying to investigate reports of a new massacre that raised the pressure on world powers struggling to halt the carnage and save a peace plan from collapse.

U.N. Secretary-General Ban Ki-moon described as 'unspeakable barbarity' the reported killing of at least 78 villagers by forces loyal to Syrian President Bashar al-Assad and warned that a civil war was imminent.

Speaking at a special session of the U.N. General Assembly on Syria, international envoy Kofi Annan acknowledged his U.N.-Arab League peace plan was not working and said there must be 'consequences' for those who do not comply.

Ban said hopes for consolidating the peace plan were fading and Annan himself warned the U.N. Security Council that the crisis in Syria could soon spiral out of control, diplomats said. Annan, Ban's predecessor as U.N. secretary-general, called for "substantial pressure" on Damascus to stop the violence.

"The Syrian people are bleeding," Ban said after the council meeting. "They are angry. They want peace and dignity. Above all, they all want action. "The danger of a civil war is imminent and real."

Opposition activists said up to 40 women and children were among those killed in the Sunni Muslim village of Mazraat al-Qubeir on Wednesday, posting film on the Internet of bloodied or charred bodies.

"There was smoke rising from the buildings and a horrible smell of human flesh burning," said a Mazraat al-Qubeir resident who told how he had watched Syrian troops and 'shabbiha' gunmen attack his village as he hid in his family's olive grove.

"It was like a ghost town," he told Reuters by telephone, asking not to be identified because he feared for his safety.

"After the shabbiha and tanks left, the first thing I did was run to my house. It was burned. All seven people from my house were killed. I saw bodies on the stairs, the bathroom and bedroom. They were all burned," the witness said.

The latest killings, less than two weeks after 108 men, women and children were slain in the town of Houla, added to pressures on world powers to stop the bloodshed in Syria.

They have been paralysed by rifts pitting Western and most Arab states against Assad's defenders in Russia, China and Iran.

Despite growing pressure on Moscow, those rifts appeared no closer to resolution on Thursday as leaders of a bloc grouping China, Russia and Central Asian states called for dialogue to resolve the Syria conflict, rather than any firmer action by the Security Council.

The latest reports from the ground cast a long shadow over a day of consultations and debate on Syria at the United Nations.

"Today's news reports of another massacre in (Mazraat) al-Qubeir ... are shocking and sickening," Ban told the 193-nation assembly.

"A village apparently surrounded by Syrian forces. The bodies of innocent civilians lying where they were, shot. Some allegedly burned or slashed with knives.''

Ban said U.N. monitors, in Syria to check compliance with a truce declared by Annan on April 12 but never implemented, had come under small-arms fire on their way to Mazraat al-Qubeir.

There was no mention of any of the monitors being injured.

The chief of the monitoring mission, General Robert Mood, said Syrian troops and civilians had barred the team, stopping them at checkpoints and turning them back. Officials said the monitors would try again on Friday to visit the site.

A Syrian official denied reports from the village, telling the state news agency that residents had asked security forces for help after 'terrorists' killed nine women and children.

Assad, who has yet to comment on Wednesday's violence, decried the Houla killings as 'monstrous' and denied his forces were responsible.

Video purportedly from Mazraat al-Qubeir showed the bodies of at least a dozen women and children wrapped in blankets or white shrouds, as well as the remains of burned corpses.

"These are the children of the Mazraat al-Qubeir massacre. ... Look, you Arabs and Muslims, is this a terrorist?" asks the cameraman, focusing on a dead infant's face. "This woman was a shepherd and this was a schoolgirl."

A Hama-based activist using the name Abu Ghazi listed more than 50 names of victims, many from the al-Yateem family, but said some burned bodies could not be identified. The bodies of between 25 and 30 men were taken away by the killers, he said.

Shabbiha, drawn mostly from Assad's minority Alawite sect that is an offshoot of Shi'ite Islam, have been blamed for the killings of civilians from the Sunni Muslim majority.

That has raised fears of an Iraq-style sectarian bloodbath and worsened tensions between Shi'ite Iran and mainly Sunni-led Arab states.

British Foreign Secretary William Hague cited evidence of 'escalating criminality' by pro-government forces.

"Syria is clearly on the edge ... of deeper violence, of deep sectarian violence; village against village, pro-government militias against opposition areas and of looking more like Bosnia in the 1990s than of Libya last year," he told reporters.

Events in Syria's 15-month-old uprising are difficult to verify due to tight state curbs on international media access.

YEMEN-STYLE TRANSITION?

U.S. Secretary of State Hillary Clinton said the United States was willing to work with all U.N. Security Council members, including Russia, on a conference on Syria's political future, but made clear that Assad must go and his government be replaced with a democratic one.

She planned to meet with Annan on Friday in Washington.

Moscow has used its U.N. Security Council veto and other tools to protect Assad, who has given Russia a firm foothold in the Middle East and is a buyer of Russian weapons.

A senior Russian diplomat said Moscow would accept a Yemen-style power transition in Syria if it were decided by the people, referring to a deal under which Yemeni leader Ali Abdullah Saleh stepped down in February after a year of unrest.

"The Yemen scenario was discussed by the Yemenis themselves. If this scenario is discussed by Syrians themselves and is adopted by them, we are not against it," Deputy Foreign Minister Mikhail Bogdanov said, according to the Interfax news agency.

Russian Foreign Ministry spokesman Alexander Lukashevich said Russia 'most decisively condemns the barbarous acts of violence' reported on Wednesday, but he did not assign blame.

Rebel groups in Syria say they are no longer bound by Annan's truce plan and want foreign weapons and other support.

Western leaders, wary of new military engagements in the Muslim world, have offered sympathy but shown no appetite for taking on Assad's military, supplied by Russia and Iran.

Speaking at the U.N. General Assembly, Russian Ambassador Vitaly Churkin warned countries arming Syria's opposition that such weapons could end up in the hands of 'terrorists'.