Opposition activists in Syria said Friday that government forces rained shells on a poor, farming village before pro-regime gunmen moved in, killing scores of people in what rebels claim could be one of the worst single days of bloodshed in the uprising against President Bashar Assad.

The accounts of the killings and death tolls varied widely. Some activists said they had the names of around 30 people killed; others said 200 had been slain but could not provide names.

If confirmed, the attack on Tremseh would mark the latest in a string of brutal offensives by Syrian forces attempting to crush the rebellion.

The head of the U.N. monitoring mission in Syria blamed government forces, saying they attacked from the air and ground in "continuous violence."

The killings were likely to fuel debate among world powers that remain sharply divided on what to try next to stop Syria's violence. All previous efforts, including a peace plan brokered by international envoy Kofi Annan, have failed to quell the bloodshed.

Yet much remains unclear about what happened in the isolated central Syrian village, including why it was targeted and whether all of the dead were civilians. One activist group said dozens of the dead were rebel fighters.

Amateur videos showed the bodies of 17 people said to have been killed. Local activists, who gave the high death toll, could not provide lists of names, saying they were still being compiled.

One clip showed a young man wailing over the body of an elderly grey-haired man wrapped in a blanket and lying in the street.

"Come on, Dad. For the sake of God, get up," the man sobs. A boom is heard in the background. Activist claims and videos could not be independently verified.

But the violence was certain to cast new doubts over about whether the international community's diplomatic efforts to end the crisis remain relevant. Kofi Annan, the international envoy whose peace plan for Syria has been largely ignored by all sides, said he was "shocked and appalled" by the reports of the attack.

He singled out the government for using heavy weaponry in populated areas, something it was supposed to have stopped doing three months ago.

Maj. Gen. Robert Mood, head of U.N. monitoring mission, told reporters in Damascus that a group of observers about five kilometers (three miles) away during the violence confirmed the use of heavy weaponry and attack helicopters.

He said his team was ready to investigate if a cease-fire is reached.

The nearly 300 observers in Syria have largely stopped moving around because of continued violence. Government forces have also prevented them from visiting sites of past massacres.

Syria's violence has grown increasingly chaotic in recent months and the protests calling for political change that began in March 2011 have largely been overshadowed by the scores of rebel groups now waging an armed insurgency against the government.

Rebel groups operate independently of each other, and a string of large suicide bombings has raised suspicions that al-Qaida fighters are operating in Syria.

Activists often blame attacks on shabiha, or pro-government thugs who do not directly answer to any military structure -- allowing the government to deny responsibility for their actions.

Independent investigation is nearly impossible in Syria, which has one of the most authoritarian governments in the Middle East and bars most journalists from working in the country.

Still, a picture of violence in Tremseh began to emerge Friday.

One amateur video posted online late Thursday showed the dead bodies of 15 men lined up on a floor. Some were covered in blood and have wounds to their heads and chests. A second video showed a man's body lying on a hospital gurney.

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 has referred to those seeking its overthrow as terrorists, not people seeking reform, throughout the 16-month uprising.

The killings in Tremseh, about 15 kilometers (nine miles) northwest of the central city of Hama, reflect the difficulty of getting reliable information on events inside Syria, a country of 22 million people that is about the size of North Dakota.

Two activists reached Friday via Skype who said they were in villages near Tremseh gave similar accounts of the attack.

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

"We saw the events," he said, adding that he was a few kilometers (miles) from the village. "Lots of people tried to get the families out but they weren't able to."

After the shelling, the army entered with pro-government thugs, who gunned down and stabbed residents in the streets, he said.

Darwish said activists had compiled the names of about 200 dead, but he did not share the list. He said chaos reigned in the area as residents searched for the dead and missing.

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

"They kept shelling the city and the weapons that the Free Army had were not enough to keep them out," he said. "So they started trying to get out the wounded and the families by clashing in one place to open a way out."

He, too, put the number of dead at more than 200, but did not provide a list of names. He said many of the dead were killed when a shell collapsed the roof of a mosque where they had sought shelter.

The Britain-based Syrian Observatory for Human Rights said Friday that more than 160 people had been killed in Hama province, most of them in Tremseh, though it only had the names of about 40 of them. It said dozens of the dead were rebel fighters and that 30 bodies were totally burned. Others were stabbed.

Another video showed a tank in the street while large booms and gunfire are heard in the background.

Thursday's killing recalled a massacre in late May in the area of Houla, about 35 kilometers (22 miles) south of Tremseh, where more than 100 people were killed. Activists then also blamed the army and pro-government thugs.

A U.N. investigation said forces loyal to the government "may have been responsible" for many of the deaths. Assad has denied that, saying "terrorists" who were trying to frame the government carried out the attacks.

The Syrian government gave a very different story of the Tremseh killing, with the state news agency saying that dozens of members of "armed terrorist groups" had raided the village and were randomly firing on residents.

Security forces clashed with the armed men, killing and capturing many of them, the report said. It said three soldiers and some 50 residents were killed.

The agency provided no photos or videos.

Activists say more than 17,000 people have been killed in the revolt, most civilians. The government says more than 4,000 security forces have been killed. It does not provide numbers of civilian dead.

Syrian state-run television said a booby trapped car exploded Friday at the upscale Mazzeh district in Damascus, causing material damage but no casualties. The explosion occurred about 50 yards (meters) away from the embassy of Iran, one of Assad's main allies.