Reports of a massacre of about 200 people in the Syrian village of Tremseh drew international condemnation and protests Friday, with Secretary of State Hillary Rodham Clinton accusing the government of having “murdered” civilians.

Kofi Annan, the U.N. special envoy to Syria, said he was “shocked and appalled” by the reported massacre early Thursday, apparently the deadliest single attack since the country’s uprising began in March 2011.

There were starkly different accounts of who carried out the assault on the farming village in the central region of Hama. Opposition activists said government soldiers and militia fighters were responsible, while the government-run Syrian Arab News Agency (SANA) blamed “terrorists.”

Details of the attack could not be independently confirmed because the Syrian government has severely restricted journalists’ access.

Clinton, who is traveling in Asia, said in a statement issued in Washington that the reported use of artillery, tanks and helicopters in the attack provided “indisputable evidence that the regime deliberately murdered innocent civilians,” according to Reuters news service.

“Those who committed these atrocities will be identified and held accountable,” she said.

Annan, who has been shuttling between Damascus, Tehran and Baghdad in the past week in an effort to find a peaceful resolution to the conflict, also blamed the Syrian government.

In a statement, he said that “the confirmed use of heavy weaponry” violated a Syrian government undertaking not to deploy such weapons in population centers. He added, “I condemn these atrocities in the strongest possible terms.”

In a letter Friday to U.N. Secretary General Ban Ki-moon, Annan called the Syrian government’s actions “another grim reminder that the [U.N. Security] Council’s resolutions continue to be flouted.” His call earlier in the week for the council to impose “consequences for non-compliance” on the government “could not be more urgent in light of unfolding events,” Annan wrote.

Maj. Gen. Robert Mood, head of the U.N. Supervision Mission in Syria, said in Damascus that a U.N. team had observed the fighting from three or four miles outside Tremseh, adding that it involved “mechanized units, indirect fire as well as helicopters.”

Mood added: “If we have credible cessation of violence and a local cease-fire, we stand ready to go in with a larger team to verify the facts on the ground.”

The mandate for the observer team in Syria runs out on July 20 unless the U.N. Security Council votes to extend its mission.

Speaking to council members in New York, Ban pleaded Friday for unity behind a new resolution that would extend the mandate and authorize harsh new international sanctions if the attacks do not stop, according to a senior Western diplomat who spoke on the condition of anonymity about the closed-door session. Russia has opposed a U.S.-backed resolution being debated that would set a 10-day deadline for the Syrian government to withdraw all troops and heavy weapons from urban areas.

In its account of the Tremseh killings, SANA said that “armed terrorist groups” attacked the village and fired randomly at residents and homes, “killing more than 50 persons.” It said security forces later “clashed with the terrorists, inflicting huge losses.” Three security personnel were killed, the agency said.

As reports of the massacre trickled out of Syria, the Security Council convened Thursday, with Russia and Western nations offering rival draft resolutions to deal with the crisis. Russia’s deputy ambassador to the United Nations, Igor Pankin, told reporters that any further sanctions against the Syrian government would be a “red line.”

Regardless of how the U.N. vote plays out, pressure is mounting on Syrian President Bashar al-Assad. On Wednesday, the Syrian ambassador to Iraq, Nawaf al-Fares, defected and announced his support for the opposition.

“There is no way that [Assad] can be pushed from power without force, and the Syrian people realize this,” Fares said in an interview with al-Jazeera on Thursday.

Last week, Manaf Tlas, a brigadier general and confidant of Assad, also fled the country. Tlas has yet to announce support for the opposition, but his departure represented a symbolic blow nonetheless.

The alleged massacre on Thursday appeared to be yet another sign that the conflict in Syria is taking a sectarian turn. Tremseh, a village northwest of the city of Hama, is predominantly Sunni Muslim, while the militiamen accused of staging the attack mostly belonged to Assad’s minority Alawite sect, according to opposition activists.

The attack, which one opposition group referred to as “ethnic cleansing,” started around 5 a.m. Thursday as Tremseh was shelled by government forces for about two hours, according to the Revolution Leadership Council of Damascus. Electricity and communication lines were cut off, and militiamen known as shabiha reportedly stormed the town, targeting civilians. Many civilians were killed with knives, and some bodies were burned, the council said.

Fighters of the rebel Free Syrian Army tried to defend the village but were overrun by the Syrian army, said Saleh Hamawi, an activist based in Hama who heard accounts of the attack from opposition supporters near Tremseh. “The people were screaming in the streets,” said Hamawi, who uses a pseudonym for his safety.

A video posted on YouTube shows a number of elderly women reportedly fleeing the town carrying small bags. Opposition activists claimed that several civilians were gunned down by government troops in the fields surrounding the village as they tried to flee.

The aftermath of the attack is also documented in a video posted online that shows more than a dozen bloody and mangled bodies of men laid out on a blanket as a young man chants, “Allahu akbar” (God is great).

Another video clip shows a young man sobbing over the body of a gray-haired man covered by a blanket. “Come on Dad. For the sake of God, get up,” the grieving man wails, according to the Associated Press.

Opposition activists say that Tremseh had been relatively peaceful but that there have been occasional anti-Assad protests.

A statement issued by the Free Syrian Army on Thursday called the attack a “brutal and ugly massacre” and threatened Assad’s supporters. “Assad is sinking in a sea of blood,” the statement said. “Do not sink with him. You will face death and severe punishment.”

Despite a communication blackout around Tremseh, word of the attack appears to have spread inside Syria. The Britain-based Syrian Observatory for Human Rights reported protests in Damascus and a number of smaller cities after morning prayers Friday, with demonstrators condemning the killings.

A car bomb also exploded near the Iranian Embassy in Damascus on Friday, causing “material damage,” according to Iran’s state-run Press TV. There were no immediate details on casualties. Iran has staunchly supported the Assad government during the rebellion.

The number of Syrian refugees fleeing to neighboring countries is rapidly increasing, with about 100,000 registered with the United Nations. Inside Syria, “we have used the terminology ‘appalling,’ ‘desperate’ and ‘deplorable’ — we have run out of the language to describe how it is for the civilian population,” John Ging, director of the U.N. humanitarian effort, said Friday.

Because of the “limited donor base” for humanitarian operations in Syria, only about 20 percent of a $193 million U.N. appeal has been funded, Ging said. The United States has provided about $52 million in humanitarian assistance for Syria this fiscal year, although not all of it goes through U.N. agencies.

Karen DeYoung in Washington and Suzan Haidamous and a special correspondent in Beirut contributed to this report.