The UN singled out government forces Friday for blame in the latest massacre in Syria, a frenzy of killing that raises new questions about whether diplomacy has any chance to end the crisis more than 16 months into the bloodiest revolt of the Arab Spring.

As the violence turns ever more chaotic, analysts warn the effort by special envoy Kofi Annan has become nothing more than a pretense, with government forces, rebels, jihadists and others fighting for power.

"Violence and escalation have outpaced political and international diplomacy," said Fawaz A. Gerges, director of the Middle East Center at the London School of Economics.

"I don't see a light at the end of the tunnel.... All I see is more violence and more escalation, and this horrible massacre is another sign that Syria is spiralling out of control."

Scores of people were killed Thursday when Syrian gunners bombarded the impoverished village of Tremseh with tanks and helicopters in what rebels claim was among the worst single days of bloodshed in the uprising against President Bashar Assad.

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This photo, which could not be independently verified, purports to show a wounded victim of the violence in Tremseh, Syria, where 200 were killed, according to some claims. (Shaam News Network/Associated Press)

The accounts of the killings and death tolls varied widely. Late Friday, local activists backed away from early reports that more than 200 people were killed. One said he had confirmed 74, but had only 20 names. Another provided a list of 103 names.

For its part, the Syrian government said more than 50 people were killed when Syrian forces clashed with "armed gangs" that were terrorizing village residents. The regime refers to its opponents as terrorists and gangsters.

Much remains unclear about what happened in Tremseh, an isolated hamlet in Hama province, including why it was targeted and whether all of the dead were civilians. One activist group said dozens of victims were rebel fighters.

An amateur video posted online showed a young man wailing over the body of an elderly, gray-haired man wrapped in a blanket.

"Come on, Dad. For the sake of God, get up," the man sobbed as a boom was heard in the background.

Another video showed a mass grave that was three bodies wide and about 10 bodies long. The video's narrator called it "the first group of martyrs from the Tremseh massacre."

Neither activists' claims nor the videos could be independently verified.

Anti-Assad protests

The killings fuelled debate about what to try next to stop the violence, which activists say has killed more than 17,000 people despite several rounds of sanctions and increasingly frantic condemnation by the UN, the U.S. and its Western and Arab allies.

Reflecting the deep frustration, activists held anti-regime protests across Syria on Friday under the banner "Remove Kofi Annan, the servant of Assad and Iran."

"Down with Annan, the agent of Iran!" protesters chanted in the town of Maaret al-Numan. Iran is one of the Syrian regime's strongest backers.

In a statement Friday, Annan said he was "shocked and appalled" by the reports of the attack on Tremseh, and condemned the government for using heavy weaponry in populated areas, something it was supposed to have stopped three months ago.

Maj. Gen. Robert Mood, head of the UN monitoring mission, told reporters in Damascus that a group of observers deployed about five kilometres from Tremseh confirmed the use of heavy weaponry and attack helicopters, implicating the government.

The violence has grown increasingly chaotic over the course of the uprising, which began in March 2011 with mostly peaceful protests. Government forces launched a ferocious crackdown on the demonstrations, leading many people to take up arms.

Annan to meet Russia's Lavrov

Annan is set to meet with Russian government officials on Monday. He will hold talks with Russia's foreign minister Sergei Lavrov.

Russia has refused to back parts of Annan's peace plan for Syria.

Washington has been pressuring Russia to throw its full support behind the plan, and to withdraw all support for the Assad regime.

Besides the government crackdown, rebel fighters are launching increasingly deadly attacks on regime targets, and several massive suicide attacks this year suggest al-Qaeda or other extremists are joining the fray.

Against this backdrop, diplomacy appears all but doomed to fail.

The government and the opposition — which is fractious and largely leaderless — have agreed in theory to Annan's plan, which calls for a ceasefire by both sides and for the government to pull its tanks out of population centres.

But both sides have largely ignored their promises to Annan, and the presence of other forces, such as violent extremists who are not party to any such agreement, only complicates efforts to stop the bloodshed.

Threats at the UN Security Council have no heft because veto-wielding member Russia stands staunchly on the side of the regime in Syria, its longtime ally.

World reacts to latest violence

On Friday, Secretary of State Hillary Rodham Clinton expressed outrage over the killings in Tremseh and demanded that the Security Council take action to stop the violence.

"History will judge this council," she said. "Its members must ask themselves whether continuing to allow the Assad regime to commit unspeakable violence against its own people is the legacy they want to leave."

Canadian Foreign Affairs Minister John Baird said Friday that Canada is appalled by the Tremseh massacre.

"The international community cannot stand by and allow these atrocities to continue," he said in a statement. "Canada urges all members of the UN Security Council to come to agreement on a resolution that will impose tough, binding economic sanctions against the regime.

"Those countries blocking any meaningful attempt at a resolution will be held accountable for their decisions. The senseless bloodshed in Syria is unconscionable. Those in a position to influence Syria must do their part to bring about an immediate end to the violence."

Russia, for its part, condemned the Tremseh killings, but blamed them on "terrorists" opposed to Assad.

Regime calls uprising a 'foreign plot'

The Syrian government claims the uprising is a foreign plot to weaken Syria, casting all its opponents — from pro-democracy demonstrators to armed rebels — as terrorists.

NATO and the UN have all but ruled out foreign military intervention. Syria is intertwined in alliances with Iran, Hezbollah and Palestinian militant groups, and borders Israel — making the fallout from military action unpredictable.

"The Western powers have no appetite or political will to intervene militarily," Gerges said. "So once they say there is no more diplomatic initiative, then they have to say 'What is Plan B?' There is no Plan B."

There is virtually no way to perform an independent investigation in Syria, one of the most authoritarian states in the Middle East. Assad has largely sealed off the country and prevented reporters from moving freely. Members of a team of 300 UN monitors sent to Syria to provide an unbiased look at the violence have been confined to their hotels since June 15 because of the worsening violence.

In Istanbul, the head of the Syrian National Council, an exile opposition group, called on the Security Council to meet urgently to discuss ways to protect the Syrian people, saying the latest killings raise doubts about Annan's plan.

"Kofi Annan is very much drifting away from the mission that he was entrusted with, which will make us reconsider everything he proposes," Abdelbaset Sieda told reporters.

Chilling account of massacre

Two activists reached Friday via Skype said they were in villages near Tremseh and gave a chilling account of the violence.

Bassel Darwish said the army surrounded the village early Thursday to prevent people from fleeing and pounded it with artillery, tank shells and missiles from a combat helicopter.

"Lots of people tried to get the families out but they weren't able to," he said. After the shelling, the army entered with pro-government thugs, who gunned down and stabbed residents in the streets, he said.

Initially, Darwish said activists had determined 200 people died. However, later via Skype, he sent a list of 103 names of people he said were confirmed dead.

Another activist, Abu Ghazi al-Hamwi, said local rebels tried to fight off the army but couldn't.

"They kept shelling the city and the weapons that the [rebel] Free Army had were not enough to keep them out," he said.

Al Jazeera reported an allegation that the killings happened when pro-regime forces retaliated following a Free Syrian Army attack on an army convoy. However, an activist told the Arab news network that government troops opened fire after the FSA attacked a neighbouring Alawite village.